List of dangerous human viruses. The most rare human viral diseases

MINISTRY OF SCIENCE AND EDUCATION OF UKRAINE

Human viral diseases

Completed:

10th grade student

Secondary school №94

Gladkov Evgeny

Checked by: Suprun Elena Viktorovna

Kharkov, 2004.

Diseases that are caused by viruses are easily transmitted from sick people to healthy people and spread quickly. Much evidence has been accumulated that viruses are also the cause of various chronic diseases.

These are smallpox, polio, rabies, viral hepatitis, influenza, AIDS, etc. Many viruses to which humans are susceptible infect animals and vice versa. In addition, some animals are carriers of human viruses without getting sick.

The main groups of viruses that cause diseases in humans are presented in the table:

Vaccination (vaccination, immunization) - the creation of artificial immunity to certain diseases. For this, relatively harmless antigens (protein molecules) are used, which are part of the microorganisms that cause diseases. Microorganisms can be viruses, such as measles, or bacteria.

Vaccination is one of the very best means to protect children against infectious diseases that caused serious illness before vaccinations were available. Unfounded criticism of vaccination in the press was caused by the desire of journalists to inflate sensations from individual cases of post-vaccination complications. Yes, side effects are common to all drugs, including vaccines. But the risk of getting a complication from vaccination is much smaller than the risk of the consequences of an infectious disease in unvaccinated children.

Vaccines stimulate the immune system to respond as if it were a real infection. The immune system then fights the "infection" and remembers the microorganism that caused it. Moreover, if the microbe enters the body again, it effectively fights it.

There are currently four different types of vaccines:

biosynthetic vaccines; they contain substances obtained by genetic engineering methods and causing an immune system reaction. For example, the hepatitis B vaccine, Haemophilus influenzae.

Smallpox is one of the oldest diseases. A description of smallpox was found in the Egyptian papyrus Amenophis 1, compiled 4,000 years before our era. The causative agent of smallpox is a large, complex DNA-containing virus that multiplies in the cytoplasm of cells, where characteristic inclusions are formed. Smallpox is a particularly dangerous infectious disease, characterized by a severe course, fever, a rash on the skin and mucous membranes, often leaving behind scars.

The source of infection is a sick person from the beginning of incubation to complete recovery. The virus is dispersed with droplets of mucus and saliva when talking, coughing, sneezing, as well as with urine, sputum and crusts that have fallen off the skin. Infection of healthy people occurs with inhaled air and when using utensils, linen, clothes, household items contaminated with the secretions of the patient.

Human smallpox has now been eradicated from the world through smallpox vaccination.

Polio

Poliomyelitis is a viral disease that affects the gray matter of the central nervous system. The causative agent of poliomyelitis is a small virus that does not have an outer shell and contains RNA. An effective method of combating this disease is a live polio vaccine. The main habitat of enteroviruses in nature is the human body, or rather the intestines, hence the name. The intestine is the only reservoir of many enteroviruses, from where the viruses enter the blood, internal organs, and the central nervous system.

Poliomyelitis (polios - gray, myelos - spinal cord). The name itself suggests that the virus affects the gray matter of the spinal cord. In paretic forms of poliomyelitis, motor innervation, which is responsible for muscle movement, is actually disrupted. There are atrophic paralysis, more often of the lower, less often of the upper limbs, depending on which segment of the spinal cord is affected. The disease is very severe, crippling. It has been known for a long time, Hippocrates mentions it. Unfortunately, poliomyelitis is not uncommon.

The virus was discovered in 1945.

EPIDEMIOLOGY OF POLIO: Incubation period 7-14 days. Poliomyelitis is a very highly contagious disease, the source is a sick person with an asymptomatic form, the main route of transmission is fecal-oral. The fecal-oral route of transmission is the main one in countries with a highly developed sanitary situation. In countries with a highly developed sanitary culture, the leading route of transmission is airborne. In the first week of the disease, the virus can settle in the peripharyngeal lymph nodes and with the pharyngeal mucus when coughing, sneezing, it can be released into the environment, infecting others

PATHOGENESIS. The causative agent penetrates through the mouth often through dirty hands, dishes, water. In a certain number of cases, the virus penetrates the intestinal barrier, enters the bloodstream, and viremia occurs. In some cases, the virus crosses the blood-brain barrier and enters the spinal cord, causing damage to the motor innervation. The causative agent of poliomyelitis can cause the following diseases:

aseptic meningitis

asymptomatic forms (inapparent form), when the virus is in the intestine without penetrating into the blood.

Abortive form (small disease). The virus enters the bloodstream but cannot cross the blood-brain barrier. Clinically, this disease is manifested by angina, catarrh of the upper respiratory tract.

In a small number of children, the virus crosses the blood-brain barrier and causes damage to the motor neurons of the anterior horns of the spinal cord, the so-called paralytic form. Mortality in the paralytic form is 10%, and more than half of the children develop persistent paralysis.

IMMUNITY in poliomyelitis is lifelong, type-specific. The mechanism of immunity is determined by 2 main points:

Humoral general immunity, provided by circulating in the blood immunoglobulins class M and G2),

2. Local occurs in the tissue of the intestine and nasopharynx, pharynx, ensuring the stability of these tissues by the presence of class A secretory immunoglobulins.

COXSACKIE VIRUSES. In the city of Coxsackie (America) in 1948, in a polio clinic, viruses were isolated from sick children that did not react with polyvalent polio serum. The isolated viruses were found to cause disease in newborn suckling mice. The division of Coxsackie viruses into 2 subgroups (A and B) is associated with their ability to affect tissues of newborn mice in different ways.

Subgroup A coxsackie viruses cause flaccid paralysis, and subgroup B cause spastic paralysis. Diseases that cause Coxsackie viruses: aseptic meningitis, tonsillitis, febrile illness with a rash.

Coxsackieviruses most often cause neonatal encephalomyocarditis.

ECHO VIRUSES. Е- enteric , C - cytopathogenetic, O - orpham, H - human. In the process of studying enteroviruses, viruses were found that could not be classified as enteroviruses, since, firstly, they did not react with polyvalent polio serum, and secondly, they were not able to cause diseases in suckling mice, so they could not be classified as Coxsackievirus. At first they were called orphans - orphans. Then ECHO. ECHO viruses cause aseptic meningitis, gastroenteritis in children, febrile illnesses with summer seasonality.

TREATMENT AND PREVENTION OF POLIOMYELITIS. There is no specific treatment for poliomyelitis. There are no chemotherapy drugs, antibiotics that can help with the paralytic form. Symptomatic, restorative measures are possible.

There are 2 vaccines:

The Salk vaccine, developed in 1956 and called the inactivated polio vaccine (IPV). This is a killed vaccine, it gives general humoral immunity, but does not protect the intestines. A person who is vaccinated with this vaccine will not get sick himself, but if the virus settles in the intestines of this child, he can become a carrier of the virus and infect others.

Human viruses
or "Why do we keep getting viral infections?"

You have a fever, a tickle in your throat, a runny nose begins - you get sick. Your doctor will most likely diagnose SARS, an acute respiratory viral infection. What are viruses and how do they cause disease?

Viruses infect almost all living organisms: from bacteria to plants, animals and humans.

Human viruses cause a huge number of dangerous diseases, including AIDS, bird flu, natural (so-called "black") smallpox, SARS (severe acute respiratory syndrome, severe acute respiratory syndrome) and such "ordinary" diseases as influenza, cold, rubella. Several animal viruses are also known to infect humans. Some viruses can acquire this ability under certain circumstances, such as the recently well-known H5N1 avian influenza virus.

Figure 1. The human immunodeficiency virus (HIV) that causes AIDS.

Figure 2. Influenza virus.

For the first time viruses (from the Latin virus - poison) were described by the Russian botanist D.I. Ivanovsky in 1892. The scientist found that the causative agent of tobacco mosaic passes through a filter that traps bacteria. He showed that an extract of tobacco plants infected with tobacco mosaic filtered through a porcelain filter retained the ability to cause disease in healthy plants. The filterable pathogen of foot-and-mouth disease in cattle was discovered in 1897 by the German bacteriologist F. Leffler. Until the discovery of the nature of viruses in the late 19th century, the term "virus" referred to any infectious agents that cause disease. Only since 1898, when the Dutch botanist M. Beijerinck gave the name to the causative agent of the tobacco mosaic "filtering virus", the concept of "virus" acquired the meaning that is being put into it now. Yellow fever virus was the first human virus to be discovered. Viscerophilus tropicus(manifested by fever and jaundice), discovered by the American surgeon W. Reid in 1901.

Viruses are very small, ranging in size from tens to thousands of nanometers.

The structure of viruses

A typical virus consists of genetic material presented in the form of a DNA or RNA molecule (DNA and RNA of viruses are extremely diverse in structure - single-stranded and double-stranded, closed in a ring, etc.) and packaged in a capsid - a protein shell, often containing inclusions lipid and carbohydrate molecules. Unlike cells, viruses cannot contain both DNA and RNA. Inside the capsid, proteins necessary for virus replication, such as the enzyme reverse transcriptase (RT, from reverse transcriptase), characteristic of RNA retroviruses and necessary for the formation of a DNA molecule from the viral RNA template in an infected host cell, may be found. In the simplest filamentous or rod-shaped viruses, the protein components of the capsid are bound to the nucleic acid by non-covalent bonds, forming a helical nucleoprotein structure called the nucleocapsid. In many viruses, the capsid is covered by an additional envelope called the supercapsid or peplos, which consists of the lipid membrane of the infected cell and viral proteins. In the space between the supercapsid and the capsid, there is a protein matrix. The supercapsid may have superficial projections called spikes or ash meters. By the presence or absence of a supercapsid, viruses are divided into two types: enveloped or coated virions (the vast majority of animal and human viruses) and unenveloped or uncoated virions.

Figure 4. Rinderpest virus has a helical shape.

Among viruses without an envelope, three main types are distinguished according to the shape of capsids: rod-shaped (filamentous), spherical (icosahedral) and club-shaped (combined). Rod-shaped viruses, such as the well-studied tobacco mosaic virus, have a helical type of symmetry: inside the protein shell is a helical RNA molecule. In the capsids of spherical viruses, the genetic material is not bound or weakly bound to envelope proteins. Capsids of viruses of this type often have an icosahedral type of symmetry. Spherical-type viruses include, for example, adenoviruses that cause SARS. The structure of the adenovirus capsid has a complex structure: at the vertices of the icosahedrons there are clusters of proteins - pentons, containing the so-called fibers at the base - rods with thickenings at the ends. Viruses, consisting of structures of various types (spiral, icosahedral and additional formations), belong to the club type. Viruses of this type include some bacterial viruses that have a special name - bacteriophages or simply phages (from the Greek "phagos" - devouring). Bacteriophages of this type consist of an icosahedral head with a DNA or RNA molecule inside, adjacent to a spiral tail, at the end of which there is a hexagonal flat formation with tail processes.

Figure 5. Tobacco mosaic virus.

Figure 6. Bacteriophage phiX174 has an icosahedral capsid.

Figure 7. Scheme of the structure of adenovirus. 1 - fibers, 2 - icosahedral capsid, 3 - genome.

Figure 8. Electron micrograph of two adenoviruses.

Figure 9. Scheme of the structure of a club-shaped bacteriophage.

Figure 10. Electron micrograph of a bacterial cell attacked by bacteriophages.

The shell of coated viruses, or supercapsid, is formed by a lipid bilayer from the membranes of infected cells with viral and cellular proteins embedded in it. Coated virions have certain advantages over uncoated ones. The shell gives them greater resistance to the effects of cellular enzymes and drugs. The supercapsids of coated viruses often contain surface protrusions called spikes or peplomers. Peplomers are composed of viral glycoproteins and serve to recognize and subsequently infect host cells. Coated viruses can be rod-shaped, spherical, and club-shaped like uncoated viruses, or they can be more complex, such as poxviruses (causative agents of smallpox). The genome of poxviruses is packaged with the help of proteins into a compact structure - the so-called nucleoid, which is surrounded by a membrane and tubular formations. The virus has an outer shell with a large number of proteins embedded in it.

Figure 11. Poxvirus.

Classification and reproduction of viruses

In nature, there is a great variety of different viruses. For the convenience of their study, several classification systems have been proposed. Currently, a combination of two systems is used to classify viruses: ICTV and the Baltimore classification.

ICTV classification

The ICTV virus classification system was adopted in 1966 by the International Committee on Taxonomy of Viruses (ICTV), which also maintains the taxonomic database The Universal Virus Database ICTVdB. Classification is carried out according to three main components: according to the type of genetic material of the virion (RNA or DNA), the number of nucleic acid chains (single- or double-stranded molecules), and the presence or absence of an outer shell. Minor features are also taken into account, such as host cell type, capsid shape, immunological properties, and the type of disease caused by the virus. The classification is a series of hierarchical taxa:

I. Detachment (- viruses)
II. Family (- viridae)
III. Subfamily (- virinae)
IV Genus (- virus)
V. View (- virus)

The ICTV virus classification system is relatively new. To date, only 3 taxonomic orders are known, including 56 families, 9 subfamilies and 233 genera. More than 1,550 types of viruses have been classified within the system, but more than 30,000 virus strains remain unclassified.

Baltimore classification

The Baltimore classification is the most comprehensive, because other systems based on symptomatic differences in diseases caused by viruses or on differences in virion morphology do not clearly separate viruses due to the similarity of symptoms of diseases caused by viruses of completely different structure and using different mechanisms of reproduction.

The Baltimore virus classification system, proposed in 1971 by Nobel laureate David Baltimore, divides viruses into 7 groups depending on the mechanism of formation of viral mRNA (messenger RNA molecule on which protein is synthesized) in the host cell. In order to produce viral proteins and replicate, the virus must first form mRNA in an infected cell. However, different types of viruses use different methods of mRNA formation, depending on the type of carrier of genetic information (RNA or DNA), the number of nucleic acid chains (single- or double-stranded) and the need to use reverse transcriptase RT (reverse transcriptase) - an enzyme that synthesizes dsDNA (double-stranded DNA) according to the sRNA template (single-stranded RNA). With regard to viruses, it is convenient to designate mRNA molecules as (+)onRNA (i.e., the thread along which the formation of proteins occurs, encoding the RNA chain). Accordingly, an RNA strand complementary to (+)ssRNA is conveniently referred to as (-)ssRNA (or non-coding RNA strand).

Figure 12. David Baltimore.

Figure 13. Classification of viruses according to Baltimore.

The Baltimore classification divides viruses into the following groups:

I. dsDNA viruses- viruses containing dsDNA (for example, herpes, pox and adenoviruses). Virus replication occurs as follows: mRNA molecules ((+)onRNA) are read (transcribed) from the genome of these viruses by the enzyme DNA-dependent RNA polymerase of an infected cell, on the basis of which the synthesis of viral proteins is carried out. And copying of the viral DNA genome occurs through the exploitation of the enzyme DNA-dependent DNA polymerase of the host cell. The infectious cycle ends with the packaging of viral genomes into newly synthesized protein capsids and the release of virions from the cell.

II. onDNA viruses- viruses containing ssDNA (for example, parvoviruses). When viruses enter a cell, the viral genome is first completed to a double-stranded form with the help of cellular DNA polymerase, and then according to the mechanism of viruses from group I.

III. lncRNA viruses- viruses containing lncRNA (for example, rotaviruses that cause intestinal infections). Together with viral RNA, a viral RNA-dependent RNA polymerase enters the infected cell, which ensures the synthesis of (+)onRNA molecules. In turn, (+)ssRNA ensures the production of viral proteins in the host cell and serves as a template for the synthesis of new chains of (-)ssRNA by the viral RNA polymerase. Complementary chains of (+) and (-)RNA then form a double-stranded (+-)RNA genome, which is packaged in a protein shell, forming a new generation of virions.

IV. (+)snRNA viruses- viruses containing (+)snRNA or mRNA (for example, poliomyelitis and tick-borne encephalitis viruses, hepatitis A virus, tobacco mosaic virus of plants). According to the viral mRNA that has entered the cell, the synthesis of viral proteins immediately begins, including the enzyme RNA-dependent RNA polymerase, which is capable of synthesizing RNA molecules without the participation of DNA. With the help of this enzyme, the reproduction of viral mRNA molecules begins in the cell, and ready-made virions are assembled from the accumulated viral proteins and RNA.

V. (-)sRNA viruses- viruses containing (-)snRNA (for example, influenza, measles, rabies). Viruses of this group, in addition to (-) onRNA, "carry" with them an enzyme - RNA-dependent RNA polymerase, necessary for the formation of a complementary strand of RNA ((+) onRNA) in an infected cell at the first stage of the infectious process. Further, viral proteins are formed, including RNA-dependent RNA polymerase, which ensures the multiplication of the viral genome in this cell and is packaged into newly formed virions.

VI. sRNA-RT viruses, or retroviruses- viruses containing (+)onRNA and having in their life cycle the stage of DNA synthesis according to the RNA template. This group includes some oncoviruses (viruses capable of causing malignant diseases) and viruses such as HIV (although its genome is represented by lncRNA, the DNA synthesis stage is integral to the life cycle of the virus). The viral genome encodes the enzyme reverse transcriptase, which has the properties of both RNA-dependent and DNA-dependent DNA polymerase. Getting together with viral RNA into an infected cell, reverse transcriptase ensures the synthesis of a DNA copy according to the (+)onRNA template, first in the form of (-)ssDNA, and then in the form of dsDNA. Then viral (+)onRNA, viral proteins are synthesized and ready-made virions are formed, which leave the cell for a new stage of infection.

VII. dsDNA-RT viruses- viruses containing dsDNA and having in their life cycle the stage of DNA synthesis from an RNA template (retroid viruses, such as hepatitis B virus). The dsDNA, which is part of these viruses, is copied differently than in group I viruses (in which viral DNA is copied by a DNA-dependent DNA polymerase). In this case, (+)onRNA is first synthesized from viral DNA by cellular DNA-dependent RNA polymerase, which then forms viral proteins and DNA. DNA synthesis is carried out by the enzyme reverse transcriptase RT encoded in viral DNA.

The group of microscopic infectious agents called viroids (i.e., virus-like particles) that cause many plant diseases does not quite fit into the classification system of viruses. They are circular sRNAs and lack even the simplest protein coats found in all viruses.

Interaction of viruses with cells

The life cycles of viruses can vary greatly between species.

The most typical process of a virus entering a cell begins with the attachment of a viral capsid to a virus-specific receptor (usually of a glycoprotein nature) located on the surface of the target cell membrane. Penetration into the cell of the virus attached to the membrane occurs due to endocytosis (capture of external material by the cell by the formation of membrane vesicles, or vesicles), or fusion of the cell membrane and the virus envelope. Inside the host cell, the viral capsid is destroyed under the action of cellular enzymes, releasing the viral genetic material, on the basis of which mRNA is synthesized (with the exception of the (+)snRNA of viruses) and the formation of viral proteins and replication of the viral genome are triggered. This is followed by the assembly of viral particles, followed by the modification of viral proteins, which is characteristic of many viruses. For example, for HIV, this stage, also called maturation, occurs after the virus particle is released by the infected cell. The release of ready-made virions from an infected cell is often accompanied by its lysis (destruction). Enveloped viruses are usually released from the cell by budding, during which the virus acquires its membrane envelope with embedded viral glycoproteins.

Some viruses can go into a latent state, called persistence for eukaryotic viruses and lysogeny for bacteriophages. The genome of such viruses is incorporated into the chromosome of the host cell. Then there are two options for the development of the disease. In some cases, cells containing viral genomes as part of their chromosomes divide to form daughter cells with viral genes. Under certain circumstances, viral genes begin to work actively, which leads to the formation of new virions and the death of an infected cell (in relation to bacteriophages, this process is called the lytic stage). Another option is possible, in which the viral genes in the infected cell are constantly working, producing new and new generations of virions. The infected cell dies over time. Such a scheme is typical, for example, for retroviruses.

As mentioned above, retroviruses (which include HIV) belong to group VI viruses - these are (+)snRNA-containing viruses that use the stage of DNA synthesis according to the RNA template in their life cycle. In the infected cell, the RNA of these viruses is converted by the enzyme reverse transcriptase into the form of DNA, which is integrated into the cell chromosome. Now viral genes are being actively read along with cellular ones. The resulting mRNA provides the synthesis of viral proteins, from which virions are then formed, including the viral RNA genome and reverse transcriptase. However, virions leaving the cell do not kill it, but leave it in a damaged state. A special form of chronic infection occurs, in which the working viral genome, included in the composition of the cell chromosome, is transmitted to daughter cells. When infected with oncoviruses, the infected cell can undergo malignant transformation.

Protecting the body from viral infection, HIV

On the way to target cells, viruses encounter physical obstacles, such as skin. Viruses are not able to overcome them, so they can enter the body only through the mucous membranes (respiratory tract, digestive tract, genital organs) or blood. Viruses that enter the body are attacked by immunocompetent cells. When a virus first enters the body, the immune system begins to produce antibodies against viral proteins expressed on the membranes of infected cells. Antibodies specifically bind to viral proteins and thus “mark” infected cells, predetermining their destruction by cytotoxic T-lymphocytes. It takes some time for the formation of antibodies. The antibodies produced remain in the blood, usually ensuring that the virus is recognized and destroyed the next time it enters the body. Some viruses, such as smallpox and rubella viruses, can only cause disease once, and a person who has had these diseases or is vaccinated against them never gets sick again. It should be noted that not all cases of viral infection in the host organism produce specific antibodies.

At the intracellular level, special proteins help to fight the virus - interferons, which activate the immune system and suppress the synthesis of viral proteins, thus preventing the virus from multiplying. In addition, the virus-infected host cell excretes interferon into the extracellular space, where it acts on neighboring cells, making them immune to the virus. Damage caused to a cell by a virus can activate the molecular system of internal cellular control, directing the cell to the path of apoptosis (physiological death). Many viruses, such as picornaviruses and flaviviruses, have systems that suppress interferon synthesis and avoid triggering host cell apoptosis.

The ability of the body to produce specific antibodies in response to the penetration of a pathogen into it is based on the method of protecting the population from mass infections - vaccination, which artificially stimulates the production of antibodies against a specific pathogen. Thus, through mass vaccination, the disease of natural “black” smallpox was practically eliminated all over the world.

Many viruses change their shell proteins, which makes it impossible for them to be recognized by the cells of the immune system and leads to the development of the disease when the virus enters the body again. For example, the envelope of the influenza virus contains two characteristic proteins, hemagglutinin (H) and neuraminidase (N). Due to these proteins, the virus binds to the target cell and antibodies recognize it by the same proteins. Changes in hemagglutinin and neuraminidase make viruses unrecognizable to antibodies, and viruses freely infect target cells, resulting in disease. The constant variability of the influenza virus has repeatedly led to pandemics (world-wide epidemics) of influenza: the “Spanish flu” of the early 20th century, the pandemics of 1957 and 1968. Pandemic-causing strains are named after key viral envelope proteins: H0N1, H2N2, H3N2, and the currently dangerous avian influenza strain H5N1. The variability of viruses is a major obstacle to the development of effective antiviral vaccines and drugs.

HIV infection occurs in a unique way that allows the virus to escape attack by the immune system. The virus uses the "best defense - attack" strategy, hitting the cells of the immune system itself. The progression of the infectious process during HIV infection occurs sequentially. Having penetrated into the body through the mucous membrane of the genital tract or directly through the bloodstream, HIV recognizes cells that have certain receptors on their surface - CD4 proteins (CD4 + cells), and takes root in them. CD4 cells include cells of the immune system such as CD4+ T-lymphocytes, monocytes, macrophages, and cells from other organs and systems of the body, including promyelocytes, megakaryocytes, lymph node dendritic cells, brain glial cells, endothelial cells of capillaries of the brain and cervix, and some others.

The interaction of HIV with the CD4 receptor of the target cell is provided by the GP120 glycoprotein located on the surface of the virus. After the fusion of the cell membranes and the virion fixed on it, the viral content enters the cell. Based on the viral lncRNA template, the enzyme reverse transcriptase synthesizes a lncDNA copy, which penetrates into the nucleus of the host cell and integrates into the cell genome. This may be followed by a latent phase and an activation phase, when active replication of the virus occurs, assembly of new viral particles and their release from the cell. Once in the bloodstream, the viruses infect other CD4+ cells. This process continues until the number of CD4+ cells in the patient's body drops to a critical level. The decrease in the number of CD4+ cells is accompanied by the progression of the clinical symptoms of the disease. A few years after infection, the patient's resistance to certain infections decreases, the likelihood of malignant tumors increases, and AIDS (acquired immunodeficiency syndrome) gradually develops. AIDS strikes at the cells of the nervous system, blood cells, cardiovascular, musculoskeletal, endocrine and other systems. Without treatment, AIDS is fatal in less than a year, and with antiretroviral therapy, patients live for more than 5 years.

Figure 16. Model of HIV

Diseases caused by viruses

Among the most common viral diseases, respiratory infections or SARS are notorious for everyone. There is not a single person who has not had ARVI at least once. Currently, more than 300 different subtypes of viruses belonging to 5 main groups and causing ARVI are known: these are parainfluenza, influenza, adenoviruses, rhinoviruses and reoviruses. All of them are highly contagious, as they are transmitted by airborne droplets and contagiously (for example, through a handshake), and are resistant to antibiotics. Susceptibility to SARS is very high, especially in young children. The main symptoms of SARS are runny nose, cough, sneezing, headache, sore throat, fatigue, high fever. Although there are many similarities in the clinical course of various respiratory infections, diseases caused by certain types of viruses have their own characteristics. Influenza is characterized by an acute onset, high fever, the possibility of developing severe forms of the disease and complications, with parainfluenza the course is milder than that of influenza, but inflammation of the larynx develops with the risk of strangulation in children. Adenovirus infection is characterized by a less pronounced onset than influenza, but there is a risk of developing tonsillitis and damage to the lymph nodes, conjunctiva of the eyes, a severe runny nose develops, and liver damage is possible. Respiratory syncytial virus infection is characterized by a milder and longer duration than influenza. Infection with this virus leads to damage to the bronchi and bronchioles, which is fraught with the development of bronchopneumonia.

Animal viruses can become dangerous to humans if they mutate and recombine. Thus, bird flu leads to the rapid death of poultry, and in wild birds it may not cause illness, despite its carriage. The SARS virus that caused a sensation a few years ago turned out to be a mutated version of the Chinese civet virus. Not to mention HIV, the primary form of which is the simian immunodeficiency virus (SIV).

Viruses of completely different types can cause diseases with almost the same symptoms. For example, hepatitis (inflammation of the liver) can be caused by hepatitis A, C, and E types belonging to three different families of (+)ssRNA-containing viruses, with hepatitis C virus covered by a lipoprotein membrane, while hepatitis A and hepatitis E viruses do not have it. . Hepatitis B virus is a retroid virus belonging to group VII according to the Baltimore classification. This example illustrates the fact that viruses that have nothing in common can cause similar diseases.

On the other hand, similar viruses can cause a variety of diseases. For example, picornaviruses, which are members of the (+)snRNA family of viruses and have very similar virions with virtually identical genomes, cause poliomyelitis, myocarditis, diabetes, conjunctivitis, colds, foot and mouth disease, hepatitis, and other diseases.

Viruses from the herpes family of viruses also cause a wide variety of diseases. Herpes simplex virus type 1 (HSV1) exists in infected cells, usually in a latent form, causing characteristic skin rashes (virus) when the host's immune status changes; varicella virus ( Varicella zoster virus- VZV) is the causative agent of chicken pox, and the Epstein-Barr virus leads to infectious mononucleosis, accompanied by inflammation of the lymph nodes. Carriage of the VZV virus can be replaced by its reactivation, which leads to the emergence of another disease - herpes zoster, accompanied by the appearance on the skin of localized areas of inflammation with vesicles filled with a clear liquid.

The table below lists the 13 major groups of viruses that are pathogenic to humans.

Figure 17. Herpes virus type 8.

Pathogenic human viruses, classification and diseases they cause

The huge variety of viruses and their unique variability poses a serious challenge to the developers of antiviral agents. Each group of viruses requires an individual approach. The absence of a relationship between the pathogenic action of viruses and their distinctive features, such as the structure of the genome, shape, size and mechanism of reproduction, makes it especially necessary to study those properties of viruses that are directly related to pathogenicity.

In subsequent publications, issues related to the development of antiviral agents will be covered in detail.

References:

1. Virology: In 3 volumes / Ed. B. Fields, D. Knipe. M.: Mir, 1989.

2. Agol V.I. How viruses cause diseases // Soros Educational Journal, 1997, No. 9, p. 27-31.

3. Agol V.I. "Noise immunity" of viruses // Molecular biology. 1998. Vol. 32, no. 1. P. 54–61.

4. Agol V.I. Variety of viruses // Soros Educational Journal, 1997, No. 4, p. 11-16.

Let's analyze infections of viral origin to understand what they are, how they develop in the bodies of infected people, what are the symptoms and how to treat them.

What is a viral infection

Viral infection is a disease caused by infectious microorganisms, viruses that enter the cells of a living organism and use its mechanisms to multiply.

To perform its vital functions, it needs to colonize the host organism and gain access to the biochemical mechanisms of replication. Therefore, viruses infect the cells of living organisms, capture them and colonize them. Once inside the cell, the virus inserts its genetic code into DNA or RNA, thereby forcing the host cell to reproduce the virus.

As a rule, as a result of such infection, the cell loses its natural functions and dies (apoptosis), but manages to replicate new viruses that infect other cells. Thus, a general infection of the whole organism develops.

There are categories of viral infections that, instead of killing the host cell, change its characteristics and functions. And it may happen that in this case the natural process of cell division will be disturbed and it will turn into a cancer cell.

In other cases, the virus after infecting the cell may go into a "sleeping" state. And only after some time, under the influence of some event that violates the achieved balance, the virus awakens. It begins to multiply again and a relapse of the disease develops.

How does the virus get infected

Infection occurs when the virus gets the opportunity to penetrate the body, overcoming its natural defensive barriers. Once in the body, it multiplies either at the site of penetration, or, with the help of blood and / or lymph, gets to the target organ.

Obviously, the way in which viruses are transmitted plays an important role.

The most common are:

• Admission by the fecal-oral route;
• Inhalation;
• Insect bites and therefore the dermal route;
• Through microscopic damage to the mucous membrane of the apparatus of the genital organs of men and women;
• Through direct contact with blood (use of used syringes or toilet items);
• Vertical transmission from mother to fetus through the placenta.

How does a viral infection develop?

Development of a viral infection depends on various parameters, in particular:

• From the characteristics of the virus. Those. the ease with which it passes from one host to another, how easily it can overcome the defenses of the new host, how successfully the body resists it, and how much damage it can create.
• From the characteristics of the host's immune system. In the human body, in addition to natural physical barriers (skin, mucous membranes, gastric juice, etc.), there is an immune system. Its task is to organize internal defense and destroy potentially dangerous substances such as viruses.
• From the environmental conditions in which the host lives. There are certain factors that obviously contribute to the spread and development of infection. An example of this is natural and climatic conditions.

After infection, an immune system reaction develops, which can lead to three outcomes:

• White blood cells, in particular lymphocytes, identify the enemy, attack him and, if possible, destroy him along with the infected cells.
• The virus manages to overcome the body's defenses and the infection spreads.
• A state of equilibrium is reached between the virus and the body, which leads to chronic infection.

If the immune system manages to overcome the infection, then the lymphocytes retain the memory of the offender. Thus, if the pathogen tries to invade the body again in the future, then, based on previous experience, the immune system will quickly eliminate the threat.

It is important to note that the vaccine works on this principle. It includes inactivated viruses or parts of them, and therefore is not able to cause a real infection, but is useful for "learning" the immune system.

Most common viral infections

Each virus, as a rule, infects a specific type of cell, for example, cold viruses penetrate the cells of the respiratory tract, rabies and encephalitis viruses infect cells of the central nervous system. Below you will find the most common viral infections.

Viral infections of the respiratory tract

They are, of course, the most common and affect the nose and nasopharynx, throat, upper and lower respiratory tract.

Viruses that most commonly affect the respiratory system:

• Rhinoviruses are responsible for the common cold, which affects the epithelium of the nose, throat and upper respiratory tract. It is transmitted through nasal secretions and enters the body through the mouth, nose, or eyes. Less commonly, a cold is spread through the air.
• Orthomyxovirus, in its various variants, is responsible for influenza. There are two types of influenza viruses: A and B, and each type has many different strains. The influenza virus strain is constantly mutating, each year bringing a new virus that is different from the previous one. Influenza attacks the upper and lower respiratory tract, lungs and is spread by airborne droplets through coughs and sneezes.
• Adenoviruses respond pharyngitis and sore throat.

Viral infections upper respiratory infections are most common in adults, while lower respiratory tract viral infections are more common in newborns and children, as well as laryngitis, which is common in newborns, tracheitis, bronchitis, and pneumonia.

Viral skin infections

There are many diseases of viral origin that affect the skin, many of them affect mainly children, for example, measles, chicken pox, rubella, mumps, warts. In this area, it is of particular importance herpes viruses to which the varicella-zoster virus belongs.

There are 8 different types known, numbered 1 to 8. Particularly common are infections with type 2 herpes virus: Epstein-Barr virus, which causes monoculosis, and cytomegalovirus. Herpesvirus type 8 causes cancer in immunocompromised patients with AIDS.

Some of the viral infections described are very dangerous during pregnancy (rubella and cytomegalovirus) because they can, with a high degree of probability, cause fetal malformations and miscarriages.

All herpes viruses lead to the development of chronic infections. Viruses remain in the host organism in a latent form. But in some cases, they can “wake up” and cause relapses. A typical example is the herpes virus, which causes chickenpox. In a latent form, the virus hides in the nerve ganglia of the spine in close proximity to the spinal cord and sometimes awakens, causing inflammation of the nerve endings with severe pain, which is accompanied by the formation of a skin rash.

Viral infections of the gastrointestinal tract

Infections of the gastrointestinal tract cause rotaviruses And hepatitis virus, noroviruses. Rotaviruses are transmitted through faeces and most often affect children and adolescents, manifesting characteristic gastrointestinal symptoms: nausea, vomiting, abdominal pain and diarrhea. Hepatitis viruses are transmitted through the consumption of contaminated food. Noroviruses are transmitted by the fecal-oral route, but can also enter the respiratory tract and cause influenza-like syndromes with lesions of the gastrointestinal tract, and therefore diarrhea and vomiting.

Viral genital infections

The viruses that affect the reproductive organs of men and women include herpes virus, human papillomavirus, human immunodeficiency virus.

Special mention deserves the infamous HIV, which causes acquired immunodeficiency syndrome, which is reflected in a sharp decrease in the effectiveness of the immune system.

Viral infections and cancers

Some types of viruses, as already mentioned, do not kill the host cell, but only change its DNA. All this leads to the fact that in the future the replication process may be disrupted and a tumor may form.

The main types of viruses that can cause the development of cancer:

• papilloma virus. May lead to cervical cancer.
• HBV and HCV virus. May cause liver cancer.
• Herpes virus 8. Causes the development of Kaposi's sarcoma (skin cancer, very rare) in AIDS patients.
• Epstein-Barr virus(Infectious mononucleosis). May cause Burkitt's lymphoma.

How are viral infections treated?

Medicines used to fight viral infections are simply called antiviral drugs.

They work by blocking the replication process of the virus responsible for the infection. But, as the virus spreads throughout the cells of the body, the scope of these drugs is limited, since the structures in which they are effective are numerically limited.

In addition, they are highly toxic to body cells. All this leads to the fact that antiviral drugs are very difficult to use. The ability of viruses to adapt to the action of drugs further confuses the tangle.

The most commonly used are the following antiviral drugs:

• Acyclovir against herpes;
• Cidofovir against cytomegalovirus;
• Interferon alfa against hepatitis B and C
• Amantadine against influenza type A
• Zanamivir from influenza A and B.

Therefore the best treatment of viral infections what remains is prevention, which is based on the use of a vaccine. But even this weapon is difficult to use, given the rapidity of some viruses mutating. A typical example is the influenza virus, which mutates so quickly that an entirely new strain breaks out every year, forcing a new type of vaccine to be introduced to deal with it.

It is absolutely useless to take antibiotics for diseases caused by viruses. Antibiotics act on bacteria. They should be used only in special cases and as prescribed by the doctor, if he believes that a secondary bacterial infection has joined the viral infection.

Some infections are asymptomatic or latent. In latent infection, viral RNA or DNA is present in the cell but does not cause disease unless trigger factors appear. The latency makes it easier for the virus to spread from person to person. Herpesviruses exhibit the property of latency.

Hundreds of viruses can infect a person. Viruses that infect humans are spread primarily by humans, mainly through secretions from the respiratory tract and intestines, some through sexual contact and blood transfusions. Their distribution among people is limited by innate immunity, acquired natural or artificial immunity, sanitary and hygienic and other social activities, as well as chemoprophylaxis.

For many viruses, animals serve as the main host, and humans are only secondary or accidental. The causative agents of zoonoses, unlike specific human viruses, are geographically limited in their distribution by those conditions in which the natural cycle of infection is maintained without human intervention (the presence of the corresponding vertebrates, arthropods, or both).

The oncogenic properties of a number of animal viruses are well studied. Human T-lymphotropic viruses type 1 are associated with some leukemias and lymphomas, Epstein-Barr virus causes malignancies, for example, nasopharyngeal carcinoma, Burkitt's African lymphoma, lymphomas in immunosuppressed transplant recipients. Hepatitis B and C predispose to the development of hepatocarcinoma. Human herpesvirus type 8 predisposes to the development of Kaposi's sarcoma, primary effusion lymphoma (body cavity lymphoma), and Castleman's disease (lymphoproliferative disorders).

The long incubation period characteristic of some viral infections gave rise to the term "slow viruses". A number of chronic degenerative diseases of previously unknown etiology are now classified as slow viral infections. Among them, we note subacute sclerosing panencephalitis (measles virus), progressive rubella panencephalitis and progressive multifocal leukoencephalopathy (JC viruses). Creutzfeldt-Jakob disease and spongiform encephalopathy have features similar to slow viral infections, but are caused by prions.

Diagnostics

Few viral diseases, such as measles, rubella, neonatal roseola, erythema infectiosum, influenza, and varicella, can be diagnosed based on clinical presentation and epidemiological data alone.

It should be remembered that accurate diagnosis is needed when specific treatment is required or when an infectious agent poses a potential threat to society (eg SARS).

Rapid diagnosis is possible in specially equipped virological laboratories by cultivation, PCR, and determination of viral antigens. Electron (not light) microscopy can help. For a number of rare diseases (for example, rabies, oriental equine encephalitis, etc.), there are specialized laboratories (centers).

Prevention and treatment

Antiviral drugs.

Progress in the use of viral drugs is very rapid. Antiviral chemotherapy targets the various phases of viral replication. They can affect the attachment of a particle to the host cell membrane or prevent the release of viral nucleic acids, inhibit the cellular receptor or viral replication factors, block specific viral enzymes and proteins necessary for virus replication, but do not affect the metabolism of the host cell. Most commonly, antiviral drugs are used therapeutically and prophylactically against herpesviruses (including cytomegalovirus), respiratory viruses, and HIV. However, some drugs are effective against many types of viruses, such as HIV drugs used to treat hepatitis B.

Interferons.

Interferons are released from infected viruses or other antigens. There are many different interferons that exhibit multiple effects, including inhibition of translation and transcription of viral RNA, termination of viral replication without disruption of host cell function. Interferons are sometimes given in a polyethylene glycol-linked form (pegylated interferons), which allows for a prolonged effect.

Interferon therapy is used to treat hepatitis B and C and human papillomavirus. Interferons are indicated for the treatment of patients with chronic hepatitis B, C in combination with impaired liver function, a certain viral load and the presence of an appropriate histological picture. Interferon-2b is used to treat hepatitis B at a dose of 5 million units subcutaneously once a day or 10 million units subcutaneously 3 times a week for 16 weeks. Treatment enhances clearance of hepatitis B virus DNA and hBeAg from plasma, improves liver function and histology.

Hepatitis C is treated with ribavirin plus pegylated interferon-2b 1.5 mcg/kg subcutaneously once a week or pegylated interferon-2a 180 mcg subcutaneously once a week. Treatment can reduce the level of viral RNA, improve liver function and histological picture. Interferon-p3 intramuscularly or directly into the affected area is used in the treatment of genital warts and skin. Optimal regimens and duration of effect are unknown. The effectiveness of the use of recombinant forms of endogenous interferon alpha in hairy cell leukemia, Kaposi's sarcoma, human papillomavirus and respiratory viruses is being studied.

Side effects include fever, chills, myalgia, weakness, begin 7-12 hours after the first injection and last up to 12 hours. There may also be depression, hepatitis, and, at high doses, bone marrow suppression.

Vaccines and immunoglobulins.

Vaccines stimulate natural immunity. Viral vaccines are used against influenza, measles, mumps, polio, rabies, rubella, hepatitis B and A, shingles and yellow fever. Vaccines for adenoviruses and varicella are available, but they are only used in high-risk groups (such as conscripts).

Immunoglobulins are used for passive immunization in a limited number of cases, for example, for post-exposure prophylaxis (hepatitis, rabies). Others may be useful in the treatment of diseases.

Respiratory viruses

Viral infections often affect the upper and lower respiratory tract. Respiratory infections may be classified according to the viruses that cause them (eg, influenza), but clinical syndrome classification is usually used (eg, colds, bronchiolitis, croup). Although individual pathogens have specific clinical symptoms (eg, rhinovirus and the common cold, respiratory syncytial virus and bronchiolitis), each virus is capable of producing virtually any symptom.

The severity of the viral infection varies widely, with children and the elderly being more severe. Mortality is determined by direct causes (depending on the nature of the viral infection), as well as indirect (as a result of exacerbations of concomitant cardiovascular pathology, bacterial superinfection of the lungs, paranasal sinuses, middle ear).

Laboratory testing of pathogens (PCR, culture, serological tests) takes too much time to be useful for a particular patient, but is necessary to analyze the epidemic situation. More rapid laboratory testing is possible for influenza viruses and respiratory syncytial virus, the value of these methods in routine practice remains unclear. Diagnosis is based on clinical and epidemiological data.

Treatment

Treatment of viral respiratory infections is usually symptomatic. Antibacterial agents are ineffective against viruses, and prophylaxis against a secondary bacterial infection is not recommended: antibiotics are prescribed only when a bacterial infection has already joined. In patients with chronic pulmonary pathology, antibiotics are prescribed with less restrictions. Aspirin should not be used in children due to the high risk of Reye's syndrome. Some patients with viral infections of the upper respiratory tract have a cough that persists for many weeks after recovery. Symptoms may respond to bronchodilators and glucocorticoids.

In some cases, antiviral drugs are important. Amantadine, rimantadine, oseltamavir and zanavir are effective against influenza. Ribavirin, a guanosine analog, inhibits the replication of RNA and DNA of many viruses and can be administered to immunocompromised patients with rhinosynchytial lesions of the lower respiratory tract.

Cold

It is an acute viral infection of the respiratory tract, self-limiting and usually without fever, with inflammation of the upper respiratory tract, including rhinorrhea, cough, and sore throat. The diagnosis is clinical. Prevention is facilitated by thorough hand washing. Treatment is symptomatic.

In most cases (30-50%), the causative agent is one of more than 100 serotypes of the rhinovirus group. The common cold is also caused by viruses from the group of coronaviruses, influenza, parainfluenza, respiratory syncytial, especially in patients undergoing reinfection.

The causative agents of the common cold are associated with the time of year, more often it is spring and autumn, less often winter. Rhinoviruses are most often spread by direct contact with an infected person, but can also be transmitted through the air.

For the development of infection, the most important is the presence in the serum and secrets of neutralizing specific antibodies that reflect previous contact with this pathogen and provide relative immunity. Susceptibility to colds is not affected by the duration of cold exposure, the individual's health and nutritional status, or pathology of the upper respiratory tract (eg, enlarged tonsils and adenoids).

Symptoms and Diagnosis

The disease begins suddenly after a short incubation period (24-72 hours) with unpleasant sensations in the nose and throat, followed by sneezing, runny nose and malaise. The temperature usually remains normal, especially when the cause is rhino- and coronovirus. During the first days the discharge from the nose is watery and profuse, then becomes thicker and purulent; the mucopurulent nature of these secretions is due to the presence of leukocytes (mainly granulocytes) and not necessarily a secondary bacterial infection. Cough with scanty sputum often lasts for 2 weeks. If there are no complications, the symptoms of a cold subside after 4-10 days. In chronic diseases of the respiratory tract (asthma and bronchitis), after a cold, there are usually exacerbations. Purulent sputum and lower respiratory symptoms are not very characteristic of rhinovirus infection. Purulent sinusitis and otitis media are usually bacterial complications, but sometimes they are associated with a primary viral infection of the mucous membranes.

Diagnosis is usually clinical, without diagnostic tests. For differential diagnosis, allergic rhinitis is most important.

Treatment and prevention

There is no specific treatment. Usually used antipyretics and analgesics, which reduce fever and reduce sore throat. Nasal congestion is treated with decongestants. Topical nasal decogestants are most effective, but their use for more than 3-5 days may lead to increased nasal discharge. For the treatment of rhinorrhea, first-generation antihistamines (for example, chlorpheniramide) or ipratropium bromide (intranasal 0.03% solution 2-3 times a day) can be used. These drugs, however, should be avoided in the elderly and those with benign prostatic hyperplasia and those with glaucoma. First-generation antihistamines cause drowsiness, but second-generation (non-sedating) antihistamines are not effective for colds.

Zinc, echinacea, vitamin C are commonly used to treat colds, but their effects have not been proven.

There are no vaccines. Polyvalent bacterial vaccines, citrus fruits, vitamins, ultraviolet light, glycol aerosols, and other home remedies do not prevent colds. Hand washing and the use of surface disinfectants reduce the prevalence of infection.

Antibiotics are prescribed only when a secondary bacterial infection is attached, with the exception of patients with chronic lung diseases.

parainfluenza

Respiratory illnesses caused by several closely related viruses, ranging from the common cold to flu-like symptoms or pneumonia, and when severe with high fever, most commonly influenza. The diagnosis is clinical. Treatment is symptomatic.

Parainfluenza viruses are RNA-containing paramyxoviruses of four serologically distinct types, designated 1,2,3 and 4. These four serotypes cause diseases of varying severity but share common antigens. Serotype 4 cross-reacts with antigenic determinants of the mumps virus and can occasionally cause respiratory disease.

Limited outbreaks of parainfluenza occur in schools, nurseries, kindergartens, hospitals and other institutions. Serotypes 1 and 2 cause autumn outbreaks. The disease associated with serotype 3 is endemic and highly contagious in children under 1 year of age. Re-infection is possible, the severity of subsequent infections is reduced and their spread is limited. Thus, in immunocompetent persons, the infection is more often asymptomatic.

The upper respiratory tract is most commonly affected in children, with or without mild fever.

When parainfluenza virus type 1 is affected, croup (acute laryngo-tracheobronchitis) develops, mainly in children aged 6-36 months. Croup begins with cold symptoms, followed by fever and barking cough, hoarseness, stridor. Respiratory failure is rare but can be fatal.

Parainfluenza type 3 virus can cause pneumonia and bronchiolitis in young children. The disease requires differential diagnosis with respiratory syncytial infection, but often weaker.

Specific laboratory diagnosis is not required. Treatment is symptomatic.

Respiratory syncytial and metapneumovirus infection

Respiratory syncytial virus (RSV) and human metapneumovirus (HMV) cause seasonal damage to the lower respiratory tract, especially in young children. The severity of the disease varies from asymptomatic to severe, and clinical manifestations include bronchiolitis and pneumonia. Diagnosis is usually clinical, although laboratory testing is available. Treatment is symptomatic.

RSV, an RNA virus classified as a pneumovirus, has subgroups A and B. Human metapneumovirus (HMV), a similar but separate virus, has recently been discovered. RSV is ubiquitous, with almost all children infected by age 4. Outbreaks of the disease usually occur in winter or early spring. Immunity in recovered patients is unstable, so contagiousness reaches 40%. Still, the presence of antibodies against RSV reduces the severity of the disease. The epidemiological features of the spread of HMV are similar to those of RSV, but the severity of outbreaks is much lower. RSV is the most common cause of lower respiratory tract disease in young children.

Symptoms and Diagnosis

The most characteristic symptoms are bronchiolitis and pneumonia. In typical cases, the disease begins with fever, respiratory symptoms that progress: after a few days, shortness of breath, cough, and wheezing join. In children under 6 months of age, sleep apnea may be the first symptom. In healthy adults and older children, the disease is usually asymptomatic or occurs as a cold without fever. Severe disease develops in elderly, immunocompromised individuals suffering from concomitant pulmonary and cardiac pathologies.

RSV (possibly also PMV) should be suspected in young children with symptoms of bronchiolitis and pneumonia during the RSV season. Since antiviral treatment is generally not recommended, there is no need for laboratory diagnosis. The latter is useful for in-hospital monitoring, which makes it possible to isolate groups of children affected by a single virus. Highly sensitive RSV antigen tests are available for children; in relation to adults they are insensitive.

Treatment and prevention

Treatment is symptomatic, including oxygen inhalation and hydration therapy as needed. Glucocorticoids and bronchodilators are usually ineffective. Antibiotics are reserved for patients with ongoing fever and x-ray confirmed pneumonia. Palivizumab is not effective for treatment. Ribaverin, which has antiviral activity, is ineffective or ineffective against RSV, has toxicity, and is not recommended for long-term use, except in immunocompromised individuals.

Passive prophylaxis with monoclonal antibodies to RSV (palivizumab) reduces the rate of hospitalization in high-risk adolescents. Vaccination is economically justified for young children who may need hospitalization (i.e. less than 2 years of age) with congenital heart disease or chronic lung disease requiring medical treatment in the last 6 months, premature babies (less than 29 weeks) who have met the RSV season at the age of less than 1 year, or born in the period of 29-32 weeks of gestation and met the RSV season at the age of less than 6 months). The dose is 15 mg/kg intramuscularly. The first dose is prescribed only before the onset of the season of exacerbations. Subsequent doses are given at 1 month intervals throughout the epidemiological season, usually 5 doses.

severe acute respiratory syndrome

Predictors of lethal outcomes are age over 60 years, severe comorbidities, elevated LDH levels, and elevated absolute neutrophil counts. Treatment of SARS is symptomatic, if necessary - mechanical ventilation of the lungs. Oseltamivir, ribavirin, and glucocorticoids may be used, but there are no data on their effectiveness.

Patients with suspected SARS should be admitted to a negative pressure box. All measures must be taken to prevent the transmission of infection by respiratory and contact routes. Personnel must wear N-95 masks, goggles, gloves, gowns.

People who have been in contact with patients with SARS (eg, family members, flight attendants, medical personnel) should be warned about the symptoms of the disease. In the absence of symptoms, they can work, go to school, etc. If fever or respiratory symptoms appear, they should limit their activity and be under medical supervision. If symptoms do not progress towards SARS within 72 hours, they may be considered tolerable.

Viral diseases infect cells in which there are already violations, which is what the pathogen uses. Modern studies have proven that this happens only with a strong weakening of the immune system, which is no longer able to adequately fight the threat.

Features of viral infections

Types of viral diseases

These pathogens are usually distinguished by a genetic trait:

• DNA - human catarrhal viral diseases, hepatitis B, herpes, papillomatosis, chicken pox, lichen;
• RNA - influenza, hepatitis C, HIV, polio, AIDS.

Viral diseases can also be classified according to the mechanism of influence on the cell:

• cytopathic - the accumulated particles break and kill it;
• immune-mediated - the virus embedded in the genome sleeps, and its antigens come to the surface, putting the cell under attack by the immune system, which considers it an aggressor;
• peaceful - the antigen is not produced, the latent state persists for a long time, replication starts when favorable conditions are created;
• degeneration - the cell mutates into a tumor.

How is the virus transmitted?

The spread of a viral infection is carried out:

1. Airborne. Respiratory viral infections are transmitted by the retraction of mucus particles splattered during a sneeze.
2. Parenterally. In this case, the disease passes from mother to child, during medical manipulations, sex.
3. Through food. Viral diseases come with water or food. Sometimes they stay dormant for a long time, appearing only under external influence.

Why are viral diseases epidemic?

Many viruses spread quickly and massively, which provokes the emergence of epidemics. The reasons for this are as follows:

1. Ease of distribution. Many serious viruses and viral diseases are easily transmitted through saliva droplets inhaled. In this form, the pathogen can maintain activity for a long time, therefore it is able to find several new carriers.
2. reproduction rate. After entering the body, the cells are affected one by one, providing the necessary nutrient medium.
3. Difficulty of elimination. It is not always known how to treat a viral infection, this is due to the lack of knowledge, the possibility of mutations and the difficulties of diagnosing - at the initial stage it is easy to confuse with other problems.

Symptoms of a viral infection

The course of viral diseases may differ depending on their type, but there are common points.

1. Fever. It is accompanied by a rise in temperature to 38 degrees, without it only mild forms of SARS pass. If the temperature is higher, then this indicates a severe course. It does not last longer than 2 weeks.
2. Rash. Viral skin diseases are accompanied by these manifestations. They may look like spots, roseola, and vesicles. It is typical for childhood, in adults rashes are less common.
3. Meningitis. Occurs with an enterovirus and is more common in children.
4. Intoxication- loss of appetite, nausea, headache, weakness and lethargy. These signs of a viral disease are due to toxins released by the pathogen in the course of activity. The strength of the impact depends on the severity of the disease, it is harder for children, adults may not notice it.
5. Diarrhea. Characteristic of rotaviruses, the stool is watery, does not contain blood.

Human viral diseases - list

It is impossible to name the exact number of viruses - they are constantly changing, adding to the extensive list. Viral diseases, the list of which is presented below, are the most famous.

1. Flu and cold. Their signs are: weakness, fever, sore throat. Antiviral drugs are used, when bacteria are attached, antibiotics are additionally prescribed.
2. Rubella. The eyes, respiratory tract, cervical lymph nodes and skin are affected. It spreads by airborne droplets, accompanied by high fever and skin rashes.
3. Piggy. The respiratory tract is affected, in rare cases, the testes are affected in men.
4. Yellow fever. Harms the liver and blood vessels.
5. Measles. Dangerous to children, affects the intestines, respiratory tract and skin.
6. . Often occurs in the background of other problems.
7. Polio. Penetrates into the blood through the intestines and breathing, with brain damage, paralysis occurs.
8. Angina. There are several types, characterized by headache, high fever, severe sore throat and chills.
9. Hepatitis. Any variety causes yellow skin, dark urine and colorless feces, which indicates a violation of several bodily functions.
10. Typhoid. Rare in the modern world, affects the circulatory system, can lead to thrombosis.
11. Syphilis. After the defeat of the genital organs, the pathogen enters the joints and eyes, spreads further. It has no symptoms for a long time, so periodic examinations are important.
12. Encephalitis. The brain is affected, a cure cannot be guaranteed, the risk of death is high.

The most dangerous viruses in the world for humans

The list of viruses that pose the greatest danger to our body:

1. Hantavirus. The causative agent is transmitted from rodents, causes various fevers, mortality in which ranges from 12 to 36%.
2. Flu. This includes the most dangerous viruses known from the news, different strains can cause a pandemic, a severe course affects the elderly and young children more.
3. Marburg. Opened in the second half of the 20th century, it is the cause of hemorrhagic fever. It is transmitted from animals and infected people.
4. . It causes diarrhea, the treatment is simple, but in underdeveloped countries 450 thousand children die from it every year.
5. Ebola. As of 2015, the mortality rate is 42%, it is transmitted by contact with the fluids of an infected person. Signs are: a sharp increase in temperature, weakness, pain in the muscles and throat, rash, diarrhea, vomiting, bleeding is possible.
6. . Mortality is estimated at 50%, intoxication, rash, fever, and lymph node damage are typical. Distributed in Asia, Oceania and Africa.
7. Smallpox. Known for a long time, dangerous only to people. Rash, fever, vomiting, and headache are characteristic. The last case of infection occurred in 1977.
8. Rabies. Transmitted from warm-blooded animals, affects the nervous system. After the appearance of symptoms, the success of treatment is almost impossible.
9. Lassa. The pathogen is carried by rats, first discovered in 1969 in Nigeria. The kidneys, nervous system are affected, myocarditis and hemorrhagic syndrome begin. The treatment is difficult, the fever claims up to 5 thousand lives annually.
10. HIV. It is transmitted through contact with the fluids of an infected person. Without treatment, there is a chance to live 9-11 years, its complexity lies in the constant mutation of cell-killing strains.

Fight against viral diseases

The complexity of the fight lies in the constant change of known pathogens, making the usual treatment of viral diseases ineffective. This makes it necessary to search for new drugs, but at the present stage of development of medicine, most measures are developed quickly, before the epidemic threshold is crossed. The following approaches have been adopted:

• etiotropic - prevention of the reproduction of the pathogen;
• surgical;
• immunomodulatory.

Antibiotics for a viral infection

In the course of the disease, there is always a suppression of immunity, sometimes it is necessary to strengthen it to destroy the pathogen. In some cases, with a viral disease, antibiotics are additionally prescribed. This is necessary when a bacterial infection joins, which is killed only in this way. With a pure viral disease, taking these drugs will not only worsen the condition.

Prevention of viral diseases

1. Vaccination- effective against a specific pathogen.
2. Strengthening immunity- Prevention of viral infections in this way involves hardening, proper nutrition, support with plant extracts.
3. Precautionary measures- the exclusion of contacts with sick people, the exclusion of unprotected casual sex.