Infection(Latin infectio - infection) is a set of biological processes that arise and develop in the body when pathogenic microbes are introduced into it.

The infectious process consists of the introduction, reproduction and spread of the pathogen in the body, its pathogenic action, as well as the reaction of the macroorganism to this action.

There are three forms of infection:

1. An infectious disease characterized by a disruption in the normal functioning of the animal organism, organic, functional disorders and morphological damage to tissues. An infectious disease may not manifest clinically or be subtle; then the infection is called latent, latent. An infectious disease in this case can be diagnosed using various additional research methods.

2. Microcarriage, not associated with the illness of the animal. A balance is maintained between the micro- and macroorganism due to the resistance of the macroorganism.

3. An immunizing infection is such a relationship between a micro- and macroorganism that causes only a specific restructuring in immunity. Functional disorders do not occur, the animal organism is not a source of the infectious agent. This form is widespread but not well understood.

Commensalism- a form of cohabitation, when one of the organisms lives at the expense of the other, without causing him any harm. Commensal microbes are representatives of the normal microflora of an animal. With a decrease in the body's resistance, they can also show a pathogenic effect.

Mutualism- a form of symbiosis, when both organisms derive mutual benefit from their cohabitation. A number of representatives of the normal microflora of animals are mutualists that benefit the owner.

The pathogenicity factors of microorganisms are divided into two groups, which determine:

invasiveness of microorganisms- the ability of microorganisms to penetrate through immunological barriers, skin, mucous membranes into tissues and organs, multiply in them and resist the immune forces of the macroorganism. Invasiveness is due to the presence in the microorganism of a capsule, mucus, surrounding the cell and opposing phagocytosis, flagella, pili, responsible for attaching microorganisms to the cell, and the production of enzymes hyaluronidase, fibrinolysin, collagenase, etc.;

toxigenicity- the ability of pathogenic microorganisms to produce exo- and endotoxins.

Exotoxins- products of microbial synthesis released by the cell into the environment. These are proteins with high and strictly specific toxicity. It is the action of exotoxins that determines the clinical signs of an infectious disease.

Endotoxins are part of the bacterial cell wall. They are released when the bacterial cell is destroyed. Regardless of the microbe-producer, endotoxins cause the same type of pathological process: weakness, shortness of breath, diarrhea, hyperthermia develop.

The pathogenic effect of viruses is associated with their reproduction in the cell of a living organism, leading to its death or to the elimination of its functional activity, but an abortive process is also possible - the death of the virus and the survival of the cell. Interaction with the virus can lead to cell transformation and tumor formation.

Each infectious agent has its own spectrum of pathogenicity, i.e. the range of susceptible animals where microorganisms realize their pathogenic properties.

There are obligately pathogenic microbes. The ability to cause an infectious process is their constant species feature. There are also facultative pathogenic (conditionally pathogenic) microorganisms, which, being commensals, cause infectious processes only when the resistance of their host is weakened. The degree of pathogenicity of microorganisms is called virulence. This is an individual feature of a specific, genetically homogeneous microbe strain. Virulence may vary depending on the conditions of existence of microorganisms.

In the case of acute infectious diseases, when pathogens enter the body of a hardy animal, as a rule, the animal becomes ill.

Such pathogens fully satisfy the three conditions of the postulate of Henle and Koch:

1. The microbe-causative agent should be detected in this disease and not occur either in healthy people or in patients with other diseases.

2. The microbe-causative agent must be isolated from the patient's body in pure form.

3. A pure culture of the isolated microbe must cause the same disease in a susceptible animal.

At present, this triad has largely lost its significance.

A certain group of pathogens does not satisfy the Koch triad: they are isolated from healthy animals and from patients with other infectious diseases. They are of low virulence, and experimental reproduction of the disease in animals fails. The causal role of these pathogens is difficult to establish.

Types of infection. Depending on the method of infection, the following types of infection are distinguished:

exogenous - the causative agent of the infection enters the body from the environment;

endogenous, or autoinfection, - occurs when the protective properties of the body are weakened and the virulence of opportunistic microflora increases.

Depending on the spread of microorganisms in the body of animals, the following types of infection are distinguished:

local, or focal, infection - the causative agent of the disease multiplies at the site of introduction into the body;

generalized - the causative agent of the disease from the place of introduction spreads throughout the body;

toxic infection - the pathogen remains at the site of introduction into the body, and its exotoxins enter the bloodstream, causing a pathogenic effect on the body (tetanus, infectious enterotoxemia);

toxicosis - exotoxins of microorganisms enter the body with food, they play the main pathogenetic role;

bacteremia / viremia - pathogens from the site of introduction penetrate into the blood and are transported by blood and lymph to various organs and tissues and multiply there;

septicemia / sepsis - reproduction of microorganisms occurs in the blood, and the infectious process is characterized by seeding of the whole organism;

pyemia - the pathogen spreads by the lymphogenous and hematogenous route to the internal organs and multiplies in them not diffusely (bacteremia), but in separate foci, with accumulation of pus in them;

septicopyemia is a combination of sepsis and pyemia.

The causative agent can cause various forms of infectious disease, depending on the ways in which microbes enter and spread in the body of animals.

Dynamics of the infectious process. Infectious diseases differ from non-infectious ones in specificity, contagiousness, staging of the course and the formation of post-infectious immunity.

Specificity - an infectious disease is caused by a certain type of microorganism.

Contagiousness - the ability of an infectious disease to spread by transmitting the pathogen from a sick animal to a healthy one.

The staging of the course is characterized by incubation, prodromal (preclinical) and clinical periods, the outcome of the disease.

The period from the moment the microbe enters the body of the animal until the first symptoms of the disease appear is called the incubation period. It is not the same and ranges from one or two days (flu, anthrax, botulism) to several weeks (tuberculosis), several months and years (slow viral infections).

In the prodromal period, the first nonspecific symptoms of the disease appear - fever, anorexia, weakness, depression, etc. Its duration is from several hours to one or two days.

Infection is the penetration and reproduction of a pathogenic microorganism (bacteria, virus, protozoan, fungus) in a macroorganism (plant, fungus, animal, human) that is susceptible to this type of microorganism. A microorganism capable of infection is called an infectious agent or pathogen.

Infection is, first of all, a form of interaction between a microbe and an affected organism. This process is extended in time and proceeds only under certain environmental conditions. In an effort to emphasize the temporal extent of infection, the term "infectious process" is used.

Infectious diseases: what are these diseases and how do they differ from non-communicable diseases

Under favorable environmental conditions, the infectious process takes on the extreme degree of its manifestation, in which certain clinical symptoms appear. This degree of manifestation is called an infectious disease. Infectious diseases differ from non-infectious pathologies in the following ways:

• The cause of the infection is a living microorganism. The microorganism that causes a particular disease is called the causative agent of that disease;
• Infections can be transmitted from an affected organism to a healthy one - this property of infections is called contagiousness;
• Infections have a latent (latent) period - this means that they do not appear immediately after the pathogen enters the body;
• Infectious pathologies cause immunological changes - they excite an immune response, accompanied by a change in the number of immune cells and antibodies, and also cause infectious allergies.

Rice. 1. Assistants of the famous microbiologist Paul Ehrlich with laboratory animals. At the dawn of the development of microbiology, a large number of animal species were kept in laboratory vivariums. Now often limited to rodents.

Infectious disease factors

So, for the occurrence of an infectious disease, three factors are necessary:

1. pathogen microorganism;
2. The host organism susceptible to it;
3. The presence of such environmental conditions in which the interaction between the pathogen and the host leads to the onset of the disease.

Infectious diseases can be caused by opportunistic microorganisms, which are most often representatives of the normal microflora and cause the disease only when the immune defense is reduced.

Rice. 2. Candida - part of the normal microflora of the oral cavity; they cause disease only under certain conditions.

And pathogenic microbes, being in the body, may not cause a disease - in this case, they speak of the carriage of a pathogenic microorganism. In addition, laboratory animals are far from always susceptible to human infections.

For the occurrence of an infectious process, a sufficient number of microorganisms that enter the body, which is called the infectious dose, is also important. The susceptibility of the host organism is determined by its biological species, sex, heredity, age, nutritional adequacy and, most importantly, the state of the immune system and the presence of concomitant diseases.

Rice. 3. Plasmodium malaria can spread only in those territories where their specific carriers live - mosquitoes of the genus Anopheles.

Environmental conditions are also important, in which the development of the infectious process is maximally facilitated. Some diseases are seasonal, some microorganisms can only exist in certain climates, and some require vectors. Recently, the conditions of the social environment have come to the fore: economic status, living and working conditions, the level of development of health care in the state, and religious characteristics.

Infectious process in dynamics

The development of infection begins with an incubation period. During this period, there are no manifestations of the presence of an infectious agent in the body, but infection has already occurred. At this time, the pathogen multiplies to a certain number or releases a threshold amount of the toxin. The duration of this period depends on the type of pathogen.

For example, with staphylococcal enteritis (a disease that occurs when eating contaminated food and is characterized by severe intoxication and diarrhea), the incubation period takes from 1 to 6 hours, and with leprosy it can stretch for decades.

Rice. 4. The incubation period of leprosy can last for years.

In most cases, it lasts 2-4 weeks. Most often, the peak of infectivity occurs at the end of the incubation period.

The prodromal period is the period of precursors of the disease - vague, non-specific symptoms, such as headache, weakness, dizziness, change in appetite, fever. This period lasts 1-2 days.

Rice. 5. Malaria is characterized by fever, which has special properties in various forms of the disease. The shape of the fever suggests the type of Plasmodium that caused it.

The prodrome is followed by the peak of the disease, which is characterized by the appearance of the main clinical symptoms of the disease. It can develop both rapidly (then they talk about an acute onset), or slowly, sluggishly. Its duration varies depending on the state of the body and the capabilities of the pathogen.

Rice. 6. Typhoid Mary, who worked as a cook, was a healthy carrier of typhoid bacilli. She infected more than 500 people with typhoid fever.

Many infections are characterized by an increase in temperature during this period, associated with the penetration into the blood of the so-called pyrogenic substances - substances of microbial or tissue origin that cause fever. Sometimes the rise in temperature is associated with the circulation in the bloodstream of the pathogen itself - this condition is called bacteremia. If at the same time the microbes also multiply, they speak of septicemia or sepsis.

Rice. 7. Yellow fever virus.

The end of the infectious process is called the outcome. The following options exist:

• Recovery;
• Lethal outcome (death);
• Transition to a chronic form;
• Relapse (recurrence due to incomplete cleansing of the body from the pathogen);
• The transition to a healthy microbe carrier (a person, without knowing it, carries pathogenic microbes and in many cases can infect others).

Rice. 8. Pneumocysts are fungi that are the leading cause of pneumonia in immunocompromised people.

Classification of infections

Rice. 9. Oral candidiasis is the most common endogenous infection.

By the nature of the pathogen, bacterial, fungal, viral and protozoal (caused by protozoa) infections are isolated. According to the number of pathogen types, there are:

• Monoinfections - caused by one type of pathogen;
• Mixed, or mixed infections - caused by several types of pathogens;
• Secondary - arising against the background of an already existing disease. A special case is opportunistic infections caused by opportunistic microorganisms against the background of diseases accompanied by immunodeficiencies.

According to their origin, they are:

• Exogenous infections, in which the pathogen penetrates from the outside;
• Endogenous infections caused by microbes that were in the body before the onset of the disease;
• Autoinfections - infections in which self-infection occurs by transferring pathogens from one place to another (for example, oral candidiasis caused by the introduction of a fungus from the vagina with dirty hands).

According to the source of infection, there are:

• Anthroponoses (source - man);
• Zoonoses (source - animals);
• Anthroposoonoses (the source can be either a person or an animal);
• Sapronoses (source - environmental objects).

According to the localization of the pathogen in the body, local (local) and general (generalized) infections are distinguished. According to the duration of the infectious process, acute and chronic infections are distinguished.

Rice. 10. Mycobacterium leprosy. Leprosy is a typical anthroponosis.

The pathogenesis of infections: a general scheme for the development of the infectious process

Pathogenesis is a mechanism for the development of pathology. The pathogenesis of infections begins with the penetration of the pathogen through the entrance gate - mucous membranes, damaged integuments, through the placenta. Further, the microbe spreads throughout the body in various ways: through the blood - hematogenously, through the lymph - lymphogenously, along the nerves - perineurally, along the length - destroying the underlying tissues, along the physiological paths - along, for example, the digestive or genital tract. The place of the final localization of the pathogen depends on its type and affinity for a particular type of tissue.

Having reached the place of final localization, the pathogen has a pathogenic effect, damaging various structures mechanically, by waste products or by releasing toxins. Isolation of the pathogen from the body can occur with natural secrets - feces, urine, sputum, purulent discharge, sometimes with saliva, sweat, milk, tears.

epidemic process

The epidemic process is the process of the spread of infections among the population. Links of the epidemic chain include:

• Source or reservoir of infection;
• transmission path;
• susceptible population.

Rice. 11. Ebola virus.

The reservoir differs from the source of infection in that the pathogen accumulates in it between epidemics, and under certain conditions it becomes a source of infection.

The main ways of transmission of infections:

1. Fecal-oral - with food contaminated with infectious secretions, hands;
2. Airborne - through the air;
3. Transmissive - through a carrier;
4. Contact - sexual, by touching, by contact with infected blood, etc.;
5. Transplacental - from a pregnant mother to a child through the placenta.

Rice. 12. H1N1 influenza virus.

Transmission factors - objects that contribute to the spread of infection, for example, water, food, household items.

According to the coverage of the infectious process of a certain territory, there are:

• Endemic - infections "tied" to a limited area;
• Epidemics - infectious diseases covering large areas (city, region, country);
• Pandemics are epidemics that have the scale of several countries and even continents.

Infectious diseases make up the lion's share of all diseases that humanity faces. They are special in that with them a person suffers from the vital activity of living organisms, albeit thousands of times smaller than himself. Previously, they often ended fatally. Despite the fact that today the development of medicine has significantly reduced mortality in infectious processes, it is necessary to be alert and aware of the features of their occurrence and development.

Definition of the concept "Infection-infectious process"

Infection, infectious process (Late Latin infectio - infection, from Latin inficio - I bring something harmful, I infect), the state of infection of the body; an evolutionary complex of biological reactions arising from the interaction of an animal organism and an infectious agent. The dynamics of this interaction is called the infectious process. There are several types of infections. A pronounced form of infection is an infectious disease with a specific clinical picture (obvious infection). In the absence of clinical manifestations of infection, it is called latent (asymptomatic, latent, inapparent). The consequence of a latent infection may be the development of immunity, which is characteristic of the so-called immunizing subinfection. A peculiar form of infections is microcarriage unrelated to the previous illness.

If the route of entry of microbes into the body is not established, the infection is called cryptogenic. Often, pathogenic microbes initially multiply only at the site of introduction, causing an inflammatory process (primary affect). If inflammatory and dystrophic

changes develop in a limited area, in the place where the pathogen is localized, is called focal (focal), and when microbes are retained in the lymph nodes that control a certain area, it is called regional. With the spread of microbes in the body, a generalized infection develops. The condition in which microbes from the primary focus enter the bloodstream, but do not multiply in the blood, but are only transported to various organs, is called bacteremia. In a number of diseases (anthrax, pasteurellosis, etc.), septicemia develops: microbes multiply in the blood and penetrate into all organs and tissues, causing inflammatory and degenerative processes there. If the pathogen, spreading from the primary lesion through the lymphatic tract and hematogenously, causes the formation of secondary purulent foci (metastases) in various organs, they speak of pyemia. The combination of septicemia and pyemia is called septicopyemia. The condition in which pathogens multiply only at the site of introduction, and their exotoxins have a pathogenic effect, is called toxemia (characteristic of tetanus).

Infection can be spontaneous (natural) or experimental (artificial). Spontaneous occurs in natural conditions when the transmission mechanism inherent in this pathogenic microbe is realized, or when conditionally pathogenic microorganisms that live in the animal's body are activated (endogenous infection, or autoinfection). If a specific pathogen enters the body from the environment, they speak of an exogenous infection. An infection caused by one type of pathogen is called simple (monoinfection), and due to the association of microbes that have invaded the body, it is called associative. In such cases, synergism is sometimes manifested - an increase in the pathogenicity of one type of microbe under the influence of another. With the simultaneous course of two different diseases (for example, tuberculosis and brucellosis), the infection is called mixed. A secondary (secondary) infection is also known, which develops against the background of any primary (main), as a result of the activation of conditionally pathogenic microbes. If, after the transfer of the disease and the release of the animal's body from its pathogen, a re-disease occurs due to infection with the same pathogenic microbe, they speak of reinfection. The condition for its development is the preservation of susceptibility to this pathogen. Superinfection is also noted - a consequence of a new (repeated) infection that occurred against the background of an already developing disease caused by the same pathogenic microbe. The return of the disease, the reappearance of its symptoms after the onset of clinical recovery is called a relapse. It occurs when the animal's resistance is weakened and the causative agents of the disease that have survived in the body are activated. Relapses are characteristic of diseases in which insufficiently strong immunity is formed (for example, infectious anemia of horses).

Complete feeding of animals, optimal conditions for their maintenance and operation are factors that prevent the occurrence of infections. Factors that weaken the body, act exactly the opposite. With general and protein starvation, for example, the synthesis of immunoglobulins decreases, the activity of phagocytes decreases. An excess of protein in the diet leads to acidosis and a decrease in blood bactericidal activity. With a lack of minerals, water metabolism and digestion processes are disturbed, and the neutralization of toxic substances is difficult. With hypovitaminosis, the barrier functions of the skin and mucous membranes are weakened, and the bactericidal activity of the blood decreases. Cooling leads to a decrease in the activity of phagocytes, the development of leukopenia, and a weakening of the barrier functions of the mucous membranes of the upper respiratory tract. When the body overheats, conditionally pathogenic intestinal microflora is activated, the permeability of the intestinal wall for microbes increases. Under the influence of certain doses of ionizing radiation, all the protective-barrier functions of the body are weakened. This contributes to both autoinfection and the penetration of microorganisms from the outside. For the development of infections, typological features and the state of the nervous system, the state of the endocrine system and RES, and the level of metabolism are important. Breeds of animals are known that are resistant to certain I., the possibility of selecting resistant lines has been proven, and there is evidence of the influence of the type of nervous activity on the manifestation of infectious diseases. A decrease in the reactivity of the body with deep inhibition of the central nervous system has been proven. This explains the sluggish, often asymptomatic course of many diseases in animals during hibernation. Immunological reactivity depends on the age of the animals. In young animals, the permeability of the skin and mucous membranes is higher, inflammatory reactions and the adsorption capacity of RES elements, as well as protective humoral factors, are less pronounced. All this favors the development of specific infections in young animals caused by conditionally pathogenic microbes. However, young animals have developed a cellular protective function. The immunological reactivity of farm animals usually increases in the summer (if overheating is excluded).

Historically, the word "infection ” (lat. inficio - infect) was first introduced to refer to sexually transmitted diseases.

Infection- the totality of all biological phenomena and processes that occur in the body during the introduction and reproduction of microorganisms in it, the result of the relationship between the macro- and microorganism in the form of adaptive and pathological processes in the body, i.e. infectious process.

infectious disease- the most pronounced form of the infectious process.

The term infection or a synonym for infectious process means a set of physiological and pathological regenerative-adaptive reactions that occur in a susceptible macroorganism under certain environmental conditions as a result of its interaction with pathogenic or opportunistic bacteria, fungi and viruses that have penetrated and multiply in it and are aimed at maintaining the constancy of the internal macroorganism environment (homeostasis). A similar process, but caused by protozoa, helminths and insects - representatives of the kingdom Animalia, is called invasion.

The occurrence, course and outcome of the infectious process are determined by three groups of factors: 1) quantitative and qualitative characteristics of the microbe - the causative agent of the infectious process; 2) the state of the macroorganism, the degree of its susceptibility to the microbe; 3) the action of physical, chemical and biological factors of the environment surrounding the microbe and macroorganism, which determines the possibility of establishing contacts between representatives of different species, the common habitat of different species, food ties, the density and number of populations, the features of the transfer of genetic information, the features of migration, etc. e. At the same time, in relation to a person, under the conditions of the external environment, first of all, one should understand social conditions his life activity. The first two biological factors are direct participants in the infectious process that develops in a macroorganism under the action of a microbe. At the same time, the microbe determines the specificity of the infectious process, and the decisive integral contribution to the form of manifestation of the infectious process, its duration, severity of manifestations and outcome is made by the state of the macroorganism, primarily the factors of its nonspecific resistance, which come to the aid of factors of specific acquired immunity. The third, environmental, factor has an indirect effect on the infectious process, reducing or increasing the susceptibility of the macroorganism, or reducing and increasing the infectious dose and virulence of the pathogen, activating the mechanisms of infection and the corresponding routes of infection transmission, etc.

Mutualism- mutually beneficial relationship (for example, normal microflora).

Commensalism- one partner (microbe) benefits without causing much harm to the other. It should be noted that in any type of relationship, a microorganism can manifest its pathogenic properties (for example, conditionally pathogenic microbes-commensals in an immunodeficient host).

pathogenicity(“disease-producing”) is the ability of a microorganism to cause a disease. This property characterizes species genetic features of microorganisms, their genetically determined characteristics, allowing to overcome the host's defense mechanisms, to manifest their pathogenic properties.

Virulence - phenotypic(individual) quantitative expression of pathogenicity (pathogenic genotype). Virulence can vary and can be determined by laboratory methods (more often DL50 - 50% lethal dose - the number of pathogenic microorganisms that can cause the death of 50% of infected animals).

According to their ability to cause diseases, microorganisms can be divided into pathogenic, conditionally pathogenic, non-pathogenic. Conditionally pathogenic microorganisms are found both in the environment and in the composition of normal microflora. Under certain conditions (immunodeficiency states, injuries and operations with the penetration of microorganisms into tissues), they can cause endogenous infections.

3) Factors of pathogenicity of microorganisms: adhesins. Factors of invasion and aggression. Tropism of microbes. Relationship between microbial cell structure and pathogenicity factors.

The main factors of pathogenicity of microorganisms- adhesins, pathogenicity enzymes, substances that inhibit phagocytosis, microbial toxins, under certain conditions - capsule, microbial motility. Virulence is associated with toxigenicity(ability to produce toxins) and invasiveness(the ability to penetrate into the tissues of the host, multiply and spread). Toxigenicity and invasiveness have independent genetic control and are often inversely related (a pathogen with high toxigenicity may have low invasiveness and vice versa).

Adhesins and colonization factors more often surface structures of a bacterial cell, with the help of which bacteria recognize receptors on cell membranes, attach to them and colonize tissues. The function of adhesion is performed pili, outer membrane proteins, LPS, teichoic acids, viral hemagglutinins. Adhesion is a trigger mechanism for the implementation of pathogenic properties of pathogens.

Factors of invasion, penetration into cells and tissues of the host. Microorganisms can multiply outside cells, on cell membranes, inside cells. Bacteria secrete substances that help to overcome the host's barriers, their penetration and reproduction. In Gram-negative bacteria, these are usually outer membrane proteins. These factors include pathogenicity enzymes.

Enzymes of pathogenicity are factors of aggression and protection of microorganisms. The ability to form exoenzymes largely determines the invasiveness of bacteria - the ability to penetrate mucous, connective tissue and other barriers. These include various lytic enzymes - hyaluronidase, collagenase, lecithinase, neuraminidase, coagulase, proteases. Their characteristics are given in more detail in the lecture on the physiology of microorganisms.

4) Bacterial toxins: exotoxins and endotoxins, nature and properties, mechanisms of action.

The most important factors of pathogenicity are considered toxins which can be divided into two large groups - exotoxins and endotoxins.

Exotoxins are produced into the external environment (host organism), usually of a protein nature, can exhibit enzymatic activity, can be secreted by both gram-positive and gram-negative bacteria. They are highly toxic, thermally unstable, and often exhibit antimetabolite properties. Exotoxins show high immunogenicity and cause the formation of specific neutralizing antibodies - antitoxins. According to the mechanism of action and point of application, exotoxins differ - cytotoxins (enterotoxins and dermatonecrotoxins), membrane toxins (hemolysins, leukocidins), functional blockers (cholerogen), exfoliants and erythrogenins. Microbes capable of producing exotoxins are called toxigenic.

Endotoxins are released only when bacteria die, are characteristic of gram-negative bacteria, are complex chemical compounds of the cell wall (LPS) - see the lecture on the chemical composition of bacteria for more details. Toxicity is determined by lipid A, the toxin is relatively heat resistant; immunogenic and toxic properties are less pronounced than those of exotoxins.

The presence of capsules in bacteria complicates the initial stages of protective reactions - recognition and absorption (phagocytosis). An essential factor of invasiveness is the mobility of bacteria, which determines the penetration of microbes into cells and into intercellular spaces.

Pathogenicity factors are controlled by:

chromosome genes;

Plasmid genes;

Genes introduced by temperate phages.

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Chapter 1. Infection, infectious process, infectious disease

Infectious diseases are widespread throughout the world, caused by various microorganisms. "Contagious" diseases have been known since ancient times, information about them can be found in the oldest written monuments: in the Indian Vedas, the works of Ancient China and Ancient Egypt. Descriptions of some infectious diseases, such as dysentery, tetanus, erysipelas, anthrax, viral hepatitis, etc., can be found in the writings of Hippocrates (460-377 BC). In the Russian chronicles, infections were described under the name of epidemics, epidemic plagues, emphasizing the main feature - mass character, high mortality and rapid prevalence among the population. Devastating epidemics and pandemics of infectious diseases have been described. It is known that in the Middle Ages an epidemic of plague (“black death”) raged, from which a third of the population of Europe died out, and all over the world from the plague in the XIV century. more than 50 million people died. During World War I, there was an influenza pandemic (“Spanish flu”) that affected 500 million people, 20 million of them died. For a long time, nothing was known about the cause of infectious diseases, it was believed that these diseases arise in connection with "miasms" - poisonous air fumes. This teaching is in the 16th century. was replaced by the doctrine of "contagia" (Fraxtoro). In the XVII-XIX centuries. many childhood infections have been described, such as measles, chicken pox, scarlet fever, etc. The full flowering of the doctrine of infectious diseases occurred in the 19th century. during the period of rapid development of microbiology and the emergence of immunology in the twentieth century. (L. Pasteur, R. Koch, I. I. Mechnikov, L. Erlich, G. N. Minkh, D. K. Zabolotny, L. A. Zilber). Advances and achievements in microbiology contributed to the isolation of infectious diseases as an independent science and the further development of the teachings on the etiology, pathogenesis, symptoms, treatment and prevention of infectious diseases. A contribution to the development of childhood infections was made by the works of A. A. Koltypin, M. G. Danilevich, D. D. Lebedev, M. S. Maslov, S. D. Nosov and other scientists.

Infectious diseases are a large group of human diseases resulting from exposure to the body of viruses, bacteria and protozoa. They develop during the interaction of two independent biosystems - a macroorganism and a microorganism under the influence of the external environment, and each of them has its own specific biological activity.

Infection is the interaction of a macroorganism with a microorganism under certain conditions of the external and social environment, as a result of which pathological, protective, adaptive, compensatory reactions develop, which are combined into an infectious process. The infectious process is the essence of an infectious disease and can manifest itself at all levels of biosystem organization - submolecular, subcellular, cellular, tissue, organ, organism.

However, not every exposure of the pathogen to the body causes disease. An infectious disease occurs if there is a violation of the function of the body and the appearance of a clinical picture. Thus, an infectious disease is an extreme degree of development of an infectious process. If, when the pathogen enters the body, no clinical picture is formed, then they speak of a healthy carriage, which can be in children with residual specific immunity or in people with congenital natural immunity. There is also convalescent carriage that occurs during the period of recovery from an infectious disease. Depending on the conditions of infection, the properties of the infectious agent, the state of the macroorganism (susceptibility, the degree of specific and nonspecific reactivity), several forms of interaction of the microorganism with the human body are described.

Manifest forms (manifested clinically) are divided into acute and chronic. There are also typical, atypical and fulminant forms, mostly ending in death. According to severity, they are divided into mild, moderate and severe forms.

In the acute form of a clinically manifested infection, the pathogen stays in the body for a short time. This form is characterized by a high intensity of release of pathogens into the environment by patients, which creates a high infectivity of patients. Many infectious diseases are acute, such as plague, smallpox, scarlet fever. Others, both acute and chronic - brucellosis, hepatitis B, dysentery.

The chronic form of the disease is characterized by a longer stay of the pathogen in the body, frequent exacerbations and remissions of the pathological process and, in case of timely treatment, a favorable outcome and recovery, as in the acute form.

Re-infection due to infection with the same infectious agent is called reinfection. If infection with another infectious agent occurs before recovery from the disease, then they speak of superinfection.

Bacteriocarrier is a process that is asymptomatic in acute or chronic form. Pathogens are present in the body, but the manifestation of the process does not occur, and outwardly the person remains healthy. Immunological changes are revealed in the body, as well as functional morphological disorders in organs and tissues, typical for this disease.

The subclinical form of infection is of great epidemiological importance, since such patients are a reservoir and source of pathogens while maintaining their ability to work and social activity, which complicates the epidemic situation. However, the high frequency of subclinical forms of certain infections (dysentery, meningococcal infection, influenza, etc.) contributes to the formation of a massive immune layer among people, which to a certain extent stops the spread of these infectious diseases.

Perelatent (latent) infection occurs as a result of prolonged asymptomatic interaction of a macroorganism with a microorganism. At its core, it is a chronic infectious disease with a benign course, occurs in diseases such as hepatitis B, herpes infection, typhoid fever, cytomegalovirus infection, and many others. etc. This form is more common in children with reduced cellular and humoral immunity, while the infectious agent is either in a defective state, or in a special stage of its life (L - form). The formation of L - forms occurs under the influence of the protective immune forces of the body and drugs (antibiotics). Atypical strains are formed with a change in all properties of the microorganism.

An essentially new form of infection interaction with the human body is a slow infection. It is characterized by a long (up to several years) incubation period - a stage at which there is no disease. At the same time, the disease progresses steadily with the development of severe disorders in many organs and systems (most often in the nervous system), and death is often observed. This type of infection includes: AIDS, congenital rubella, chronic active hepatitis with the transition to cirrhosis, etc.

Infectious diseases resulting from infection by microorganisms of the same species are called monoinfections. When infected with bacteria of different types - mixed, or mixed infection. One of the options for a mixed infection is a secondary infection, in which a new one joins an already existing disease.

The infectious process can occur due to the activation of saprophytic microflora, i.e. those microbes that constantly live on the skin and mucous membranes. In these cases, we speak of endogenous, or autoinfection, which most often occurs in debilitated children with chronic diseases, in children who have received antibacterial or cytostatic (suppressive immunity) therapy for a long time.

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Part I. Infectious diseases. basic conceptsChapter 2