Human viruses and brief information about them. Virus definition

Viruses. Surely you have heard this name many times, heard about the harm they pose to humans, heard about viral infections such as influenza, measles, smallpox, herpes, hepatitis, HIV... But what are viruses and why are they so dangerous?

All viruses are non-cellular organisms, that is, they do not have a cellular structure, and this is their main difference from other types of organisms.

The average size of viruses ranges from 20 to 300 nanometers, making them the smallest of all to which the word “living” can be applied. The average virus is about 100 times smaller than other pathogenic creatures, bacteria. The virus can only be seen with a sufficiently powerful electron microscope.

Once in the host cells, viruses begin to reproduce spontaneously, and the building material is the substance of the cell itself, which often leads to its death. This is why all viral infections are dangerous.

Interestingly, there are also beneficial viruses for humans, these are so-called bacteriophages that destroy harmful bacteria inside us.

How do viruses work?

The structure of viral particles is as simple as possible; in most cases they consist of only two components, less often - three:

genetic material in the form of DNA or RNA molecules is the actual basis of the virus, containing information for its reproduction;

capsid - a protein shell that separates and protects the genetic material from the external environment;

supercapsid - an additional lipid shell, which in some cases is formed from the membranes of donor cells.

Internal structure of a virus particle

What are viruses?

Based on their shape, all viruses can be divided into 4 large groups:

1. spiral
2. icosahedral and round
3. oblong
4. complex or incorrect

Typical forms of viruses

Viruses also spread in various ways, of which there are a huge number: by air, by direct contact, by animal carriers, through blood, etc.

Viruses (biology deciphers the meaning of this term as follows) are extracellular agents that can reproduce only with the help of living cells. Moreover, they are capable of infecting not only people, plants and animals, but also bacteria. Bacterial viruses are commonly called bacteriophages. Not so long ago, species were discovered that infect each other. They are called “satellite viruses”.

General characteristics

Viruses are a very numerous biological form, as they exist in every ecosystem on planet Earth. They are studied by a science such as virology - a branch of microbiology.

Each viral particle has several components:

Genetic data (RNA or DNA);

Capsid (protein shell) - performs a protective function;

Viruses have a fairly diverse shape, ranging from the simplest spiral to icosahedral. Standard sizes are about one hundredth the size of a small bacterium. However, most specimens are so small that they are not even visible under a light microscope.

They spread in several ways: viruses living in plants travel with the help of insects feeding on grass juices; Animal viruses are carried by blood-sucking insects. They are transmitted in a large number of ways: through airborne droplets or sexual contact, as well as through blood transfusions.

Origin

Nowadays, there are three hypotheses about the origin of viruses.

You can read briefly about viruses (our knowledge base on the biology of these organisms, unfortunately, is far from perfect) in this article. Each of the theories listed above has its own disadvantages and unproven hypotheses.

Viruses as a form of life

There are two definitions of the life form of viruses. According to the first, extracellular agents are a complex of organic molecules. The second definition states that viruses are a special form of life.

Viruses (biology implies the emergence of many new types of viruses) are characterized as organisms on the border of life. They are similar to living cells in that they have their own unique set of genes and evolve based on the method of natural selection. They can also reproduce, creating copies of themselves. Since viruses are not considered by scientists as living matter.

In order to synthesize their own molecules, extracellular agents need a host cell. The lack of their own metabolism does not allow them to reproduce without outside help.

Baltimore classification of viruses

Biology describes in sufficient detail what viruses are. David Baltimore (winner) Nobel Prize) developed his own classification of viruses, which is still successful. This classification is based on how mRNA is produced.

Viruses must make mRNA from their own genomes. This process is necessary for the replication of its own nucleic acid and the formation of proteins.

The classification of viruses (biology takes into account their origin), according to Baltimore, is as follows:

Viruses with double-stranded DNA without RNA stage. These include mimiviruses and herpeviruses.

Single-stranded DNA with positive polarity (parvoviruses).

Double-stranded RNA (rotaviruses).

Single-stranded RNA of positive polarity. Representatives: flaviviruses, picornaviruses.

Single-stranded RNA molecule of double or negative polarity. Examples: filoviruses, orthomyxoviruses.

Single-stranded positive RNA, as well as the presence of DNA synthesis on an RNA template (HIV).

Double-stranded DNA, and the presence of DNA synthesis on an RNA template (hepatitis B).

Life period

Examples of viruses in biology are found almost at every step. But everyone life cycle proceeds almost the same way. Without a cellular structure, they cannot reproduce by division. Therefore, they use materials located inside the cell of their host. Thus, they reproduce large numbers of copies of themselves.

The virus cycle consists of several stages that are overlapping.

At the first stage, the virus attaches, that is, it forms a specific bond between its proteins and the receptors of the host cell. Next, you need to penetrate the cell itself and transfer your genetic material to it. Some species also carry squirrels. Subsequently, loss of the capsid occurs and the genomic nucleic acid is released.

Human diseases

Each virus has a specific mechanism of action on its host. This process involves cell lysis, which leads to cell death. When a large number of cells die, the entire body begins to function poorly. In many cases, viruses may not cause harm to human health. In medicine this is called latency. An example of such a virus is herpes. Some latent species can be beneficial. Sometimes their presence triggers an immune response against bacterial pathogens.

Some infections can be chronic or lifelong. That is, the virus develops despite the body’s protective functions.

Epidemics

Horizontal transmission is the most common type of virus spread among humanity.

The rate of transmission of the virus depends on several factors: population density, the number of people with poor immunity, as well as the quality of medicine and weather conditions.

Body protection

The types of viruses in biology that can affect human health are innumerable. The very first protective reaction is innate immunity. It consists of special mechanisms that provide nonspecific protection. This type of immunity is not able to provide reliable and long-term protection.

When vertebrates develop acquired immunity, they produce special antibodies that attach to the virus and make it safe.

However, acquired immunity is not formed against all existing viruses. For example, HIV constantly changes its amino acid sequence, so it evades the immune system.

Treatment and prevention

Viruses are a very common phenomenon in biology, so scientists have developed special vaccines containing “killer substances” for the viruses themselves. The most common and effective method of control is vaccination, which creates immunity to infections, as well as antiviral drugs, which are capable of selectively inhibiting viral replication.

Biology describes viruses and bacteria mainly as harmful inhabitants human body. Currently, with the help of vaccination, it is possible to overcome more than thirty viruses that have settled in the human body, and even more in the body of animals.

Preventive measures against viral diseases should be carried out in a timely and efficient manner. For this, humanity must lead healthy image life and try your best possible ways boost immunity. The state must arrange quarantines in a timely manner and provide good medical care.

Plant viruses

Artificial viruses

The ability to create viruses in artificial conditions could have many consequences. The virus cannot completely die out as long as there are bodies sensitive to it.

Viruses are weapons

Viruses and the biosphere

At the moment, extracellular agents can "show off" the largest number individuals and species living on planet Earth. They perform an important function by regulating the populations of living organisms. Very often they form a symbiosis with animals. For example, the venom of some wasps contains components of viral origin. However, their main role in the existence of the biosphere is life in the sea and ocean.

One teaspoon of sea salt contains approximately a million viruses. Their main goal is to regulate life in aquatic ecosystems. Most of them are absolutely harmless to flora and fauna

But these are not all positive qualities. Viruses regulate the process of photosynthesis, therefore increasing the percentage of oxygen in the atmosphere.

>> human viral diseases

Viruses are the smallest forms of life that consist of a nucleic acid molecule, a carrier of genetic information, surrounded by a protective shell of proteins.

It is important to note that a living organism can be infected by several viruses at once. In such cases, genetic interaction between viruses and the emergence of a new recombinant form of the virus are possible. This, for example, explains the emergence of pandemic strains of the influenza virus, which are formed in the body of pigs infected simultaneously with the human and avian form of the influenza virus.

Clinical aspects of human viral diseases
Viruses play an important role in human life, as they can cause diseases of varying severity.

According to epidemiological characteristics, viral diseases are divided into anthroponotic, that is, those that only affect humans (for example, polio) and zoonotic, which are transmitted from animals to humans (for example, rabies).

The main routes of transmission of viral infection are:

1. The food route in which the virus enters the human body with contaminated food and water (viral hepatitis A, E, etc.)
2. Parenteral (or through the blood), in which virus enters directly into the blood or internal environment of a person. This mainly occurs when manipulating contaminated surgical instruments or syringes, during unprotected sexual intercourse, and also transplacentally from mother to child. In this way, fragile viruses are transmitted that quickly break down in environment(hepatitis B virus, HIV, rabies virus, etc.).
3. The respiratory tract, which is characterized by an airborne transmission mechanism, in which the virus enters the human body along with inhaled air, which contains particles of sputum and mucus expelled by a sick person or animal. This is the most dangerous route of transmission, since the virus can be transported through the air over significant distances and cause entire epidemics. This is how influenza, parainfluenza, mumps, chickenpox, etc. viruses are transmitted.

Most viruses have a certain affinity for one or another organ. For example, hepatitis viruses multiply primarily in liver cells. Based on the type of target organs that are affected during a particular disease, we distinguish the following types of viral diseases: intestinal, respiratory (respiratory), affecting the central and peripheral nervous system, internal organs, skin and mucous membranes, blood vessels, immune system, etc.

Based on the type of clinical development, we distinguish between acute and chronic viral infections. The most common are acute viral diseases that occur with severe local symptoms (damage to the mucous membrane of the respiratory tract, damage to liver tissue, damage to various areas of the brain) and general symptoms - increased body temperature, weakness, pain in the joints and muscles, changes in blood composition and etc. Acute viral infection, as a rule, ends with complete recovery of the body. In some cases, the acute form of the disease becomes chronic. Chronic viral infections occur with erased clinical picture and sometimes may not be noticed by the patients themselves. Chronic infections are difficult to treat and may progress long time, leading to significant morphological and functional changes internal organs(for example, chronic hepatitis B can lead to cirrhosis of the liver).

A separate type of viral infection is a latent infection, which is characterized by the prolonged presence of the virus in the body and complete absence symptoms of the disease. Under the influence of internal and external factors (hypothermia, decreased immunity), a latent infection can be activated and become acute.

Based on the location of the viral infection, we distinguish between local and generalized (general) viral infections. In local viral infections, the virus multiplies at the site of its entry into the body (for example, the mucous membrane of the respiratory tract) and does not penetrate into the internal environment of the body. This form of the disease is characterized by a short incubation period (the time from the moment the virus enters the body until the symptoms of the disease appear) and weak post-infectious immunity.

In generalized viral infections, the primary reproduction of the virus at the site of its entry into the body is replaced by the stage of penetration of the virus into the blood (viremia stage), with the current of which it spreads to various organs, where it causes secondary damage. Such infections are characterized by prolonged incubation period, and the immunity remaining after an illness usually protects the body from re-infection with the same virus for a long time.

Antiviral immunity
The penetration and reproduction of viruses in the human body causes a response from the immune system. The antiviral immune response consists of two components: humoral and cellular.

Humoral immunity mediated by specific antibodies that are produced by cells of the immune system in response to the presence of a virus in the body. In the first days of a viral infection, immunoglobulins (antibodies) of the IgM class are produced. In subsequent days, the secretion of IgM stops and they are replaced by antibodies of the IgG type, which have greater specificity and activity. Antibodies of the IgA type are also produced, which are released onto the surface of the mucous membranes and provide local protection against viruses. Determination of specific antibodies is an important diagnostic test that allows one to accurately determine the presence of a particular viral infection and assess the state of post-infectious immunity.

Cellular immunity carried out by T-lymphocytes and macrophages, which regulate the release of antibodies and destroy cells infected with the virus, thereby preventing its reproduction. After a viral infection, immune system cells that “remember” the virus remain in a person’s blood. When the same virus re-enters the body, these cells quickly recognize it and launch a powerful immune response - this is the essence of long-term post-infectious immunity.

However, the body’s immune response does not always bring only positive effects. Yes, when viral hepatitis B, excessive destruction of liver cells occurs precisely under the influence of activated T-lymphocytes, while the reproduction of the virus itself does not destroy liver cells.

For HIV infection characterized by deep suppression of the body's immune system. This happens because one of the targets of the virus is T-helper lymphocytes, the destruction of which leads to a complete suppression of the body's resistance.

The role of viruses in the occurrence of non-viral diseases
As mentioned above, the multiplication of the virus in the body leads to the development of one or another viral disease. However, the negative impact of viruses on the human body does not end there. In some cases, viruses cause diseases of a completely different nature.

It is now reliably known that the human papilloma virus causes cervical cancer. This occurs due to the fact that, penetrating the epithelium of the cervix, the virus activates genes responsible for the cancerous degeneration of normal cells.

In the pathogenesis of type 1 diabetes, an important role is played by viral infection, as a possible factor damaging the endocrine cells of the pancreas.

A number of pregnancy pathologies and fetal malformations are associated with various viral infections during pregnancy.

Literature:

The site provides background information for informational purposes only. Diagnosis and treatment of diseases must be carried out under the supervision of a specialist. All drugs have contraindications. Consultation with a specialist is required!

Viruses- the smallest pathogens of infectious diseases. Virus(lat. virus- “poison”) is a non-cellular infectious agent that can reproduce only inside living cells. Until the end of the 19th century. the term "virus" was used in medicine to refer to any infectious agent, causing disease. This word acquired its modern meaning after 1892, when the Russian botanist D.I. Ivanovsky established the “filterability” of the causative agent of tobacco mosaic disease (tobacco mosaic). He showed that cell sap from plants infected with this disease, passed through special filters that retain bacteria, retains the ability to cause the same disease in healthy plants. Five years later, another filterable agent - the causative agent of foot-and-mouth disease in cattle - was discovered by the German bacteriologist F. Loeffler. In 1898, the Dutch botanist M. Beijerinck repeated these experiments in an expanded version and confirmed Ivanovsky’s conclusions. He called the “filterable poisonous principle” that causes tobacco mosaic a “filterable virus.” This term has been used for many years and has gradually been shortened to one word - "virus".

In 1901, American military surgeon W. Reed and his colleagues found that the causative agent of yellow fever is also a filterable virus. Yellow fever was the first human disease identified as viral, but it took another 26 years for its viral origin to be definitively proven.

-Viruses- the simplest form of life, a microscopic particle, which is a nucleic acid molecule (DNA or RNA) enclosed in a protein shell (capsid) and capable of infecting living organisms. Their sizes range from 20 to 300 nm. The genetic material is represented by one nucleic acid molecule (DNA or RNA), not associated with proteins. The molecular weight of DNA (RNA) varies from 3·106 to 5·106. The nucleic acid of viruses can be single- or double-stranded (circular or linear). After entering the host cell, the nucleic acid of the virus, using the cell’s enzyme system, begins to replicate, synthesizing specific proteins and new viral particles.

-From others Infectious agents viruses are distinguished by a capsid. Viruses, with rare exceptions, contain only one type of nucleic acid: either DNA or RNA.

Sizes – from 15 to 2000 nm (some plant viruses). The largest among animal and human viruses is the causative agent of smallpox - up to 450 nm.

Simple viruses have an envelope - capsid, which consists only of protein subunits ( capsomeres). The capsomeres of most viruses have helical or cubic symmetry. Virions with helical symmetry are rod-shaped. Most viruses that infect plants are built according to the spiral type of symmetry. Most viruses that infect human and animal cells have a cubic type of symmetry.

Complex viruses

Complex viruses can be additionally covered with a lipoprotein surface membrane with glycoproteins that are part of the plasma membrane of the host cell (for example, smallpox viruses, hepatitis B), that is, they have supercapsid. With the help of glycoproteins, specific receptors are recognized on the surface of the host cell membrane and the viral particle attaches to it. The carbohydrate regions of glycoproteins protrude above the surface of the virus in the form of pointed rods. The additional envelope can merge with the plasma membrane of the host cell and facilitate the penetration of the contents of the viral particle deep into the cell. Additional shells may include enzymes that ensure the synthesis of viral nucleic acids in the host cell and some other reactions.

Bacteriophages have a rather complex structure. They are classified as complex viruses. For example, bacteriophage T4 consists of an expanded part - a head, a process and tail filaments. The head consists of a capsid that contains nucleic acid. The process includes a collar, a hollow shaft surrounded by a contractile sheath resembling an extended spring, and a basal plate with caudal spines and filaments.

Classification of viruses

The classification of viruses is based on the symmetry of the viruses and the presence or absence of an outer shell.

Viruses exhibit vital activity only in the cells of living organisms. Their nucleic acid is capable of causing the synthesis of viral particles in the host cell. Outside the cell, viruses do not show signs of life and are called virions.

The life cycle of the virus consists of two phases: extracellular(virion), in which it does not show signs of vital activity, and intracellular. Viral particles outside the host’s body do not lose their ability to infect for some time. For example, the polio virus can remain infectious for several days, and smallpox for months. The hepatitis B virus retains it even after short-term boiling.

The active processes of some viruses occur in the nucleus, others in the cytoplasm, and in some, both in the nucleus and in the cytoplasm.

Types of interaction between cells and viruses

There are several types of interactions between cells and viruses:

1. Productive – the nucleic acid of the virus induces the synthesis of its own substances in the host cell with the formation of a new generation.
2. Abortive – reproduction is interrupted at some stage, and a new generation is not formed.
3. Virogenic – the nucleic acid of the virus is integrated into the genome of the host cell and is not capable of reproduction.