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Infectious Disease General Concepts

Infectious diseases - an extensive group of human diseases caused by pathogenic bacteria, viruses, protozoa, etc. Infectious diseases differ from non-communicable diseases in such fundamental ways as:

• contagiousness (infectiousness);

• specificity of the pathogen;

• formation of immunity in the process of disease.

Infectious diseases can be transmitted from a sick person to a healthy one and, under certain conditions, affect large groups of people (outbreaks of disease, epidemics, pandemics).

Infection is a complex set of interactions between a pathogen and a macroorganism under certain conditions of the external and social environment, including dynamically developing pathological, protective, adaptive and compensatory reactions (combined under the name “infectious process”).

In infectious pathology, the etiology and pathogenesis of each disease is distinguished.

Etiology (from the Greek. Aitia - cause, logos - doctrine) - the doctrine of the causes and conditions of the onset and development of the disease. The term "etiology" was introduced by the ancient Greek philosopher Democritus.

Pathogenesis (from the Greek. Pathos - suffering, genesis - origin) - the doctrine of the mechanisms of the development of the disease.

The main factors of the infectious process are the pathogen, the macroorganism and the environment.

The causative agent is a microorganism that can cause an infectious process. The causative agent is the main etiological factor of an infectious disease. The most important properties of pathogens include pathogenicity, virulence, adhesiveness, invasiveness, toxigenicity.

Pathogenicity, or pathogenicity - is a species trait and represents the potential, genetically fixed ability of a microorganism of a given species to cause disease. By the presence or absence of this trait, microorganisms are classified as pathogenic, opportunistic (causing disease only under certain conditions) and non-pathogenic or saprophytes (not causing disease under any conditions).

Virulence - the degree of pathogenicity, is an individual sign of each type of pathogenic microorganism. In the experiment, it is measured by the minimum lethal dose (DLM - dosis letalis minimum). Highly virulent microorganisms, even in very small doses, can cause death. Virulence is not an absolutely stable sign and can fluctuate significantly even in different strains of the same type of pathogen.

Adhesiveness and invasiveness - the ability of pathogens to attach and penetrate into the macroorganism, due to the presence of "distribution factors" (fibrinolysin, mucinase, hyaluronidase, etc.).

Toxigenicity - the ability to synthesize and secrete toxins. There are two types of toxins:

• exotoxins are protein toxins that are produced mainly by gram-positive microorganisms and are released into the external environment by living microorganisms, have enzymatic properties, high specificity and selectively affect individual tissues and organs (for example, exotoxin botulism affects the nervous tissue);

• endotoxins are non-protein toxins produced primarily by gram-negative microorganisms, are closely associated with the microbial cell and are released only when it is destroyed, have significantly less specificity and selectivity of action.

The entry gate of infection is the place where pathogenic microorganisms enter the host through certain tissues. The entrance gates for some microorganisms are the mucous membranes of the respiratory tract (flu, tonsillitis, etc.), the digestive tract (dysentery, typhoid fever, etc.), genitals (genital infections), and for others, skin integuments (malaria, typhus). Some pathogens can enter the body in various ways (tuberculosis, viral hepatitis A, HIV infection, etc.).

From the entrance gate, the pathogen spreads through organs and tissues through the lymphatic vessels (lymphogenous pathway) or through the bloodstream (hematogenous pathway). The penetration and circulation of bacteria in the blood is called bacteremia, viruses - viremia.

Environmental conditions - affect both the pathogens of infections and the reactivity of the macroorganism.

The environment affects most microorganisms, as a rule, destructively (temperature, drying, radiation, disinfectants, antagonism of other microorganisms).

Numerous environmental factors also affect the reactivity of a macroorganism.
So, low temperature and high humidity reduce the person’s resistance to many infections, low acidity of the gastric juice - increases the likelihood of a person becoming infected with intestinal infections, etc. One should take into account the constant increase from year to year of the adverse effects of the deteriorating environmental situation.

In addition, in the human population, environmental factors are extremely important for the spread of pathogens. For example, one of the reasons for the increase in the incidence of tuberculosis in Russia is chronic stressful situations that weaken the immune system and stratification of society, which is accompanied by a change in the nutrition structure, a decrease in the population’s intake of high-grade proteins, vitamins, etc.

The forms of interaction of the infectious agent with the human body can be different and depend on the infection conditions, the biological properties of the pathogen and the characteristics of the macroorganism (susceptibility, degree of non-specific resistance and specific immunological reactivity). The interaction of macro-and microorganisms is accompanied by a varying degree of manifestation of the disease. The following forms of interaction of the pathogen with the human body are distinguished:

• manifest - clinically manifest acute and chronic forms of the disease;

• subclinical forms - characterized by the absence of clinical manifestations;

• carriage of infection (bacterial, virus and parasitic carriers) - an infectious process that is asymptomatic.

In recent years, a group of slow infections has been identified, which is characterized by a many-month or many-year incubation period, the steady progression of the disease of mainly one organ and always fatal outcome (HIV infection, spongiform encephalitis, etc.).

Periods of infectious disease. The fundamental difference between infectious diseases and others is the cyclical nature of their course, expressed in the presence of successively alternating periods:

• incubation (hidden) period - the time from the moment the pathogen enters the body until the clinical manifestation of the symptoms of the disease. Its duration is various. The incubation period is shorter, the higher the virulence and the greater the dose of the pathogen. In some diseases (flu, botulism), it counts for hours, in others (rabies, viral hepatitis B) - for weeks and even months, for slow infections - for months and years. For most infectious diseases, the incubation period is 1-3 weeks.

• the prodromal period, or the period of the precursors of the disease - the appearance of the first common signs of the disease (malaise, headache, weakness, sleep disorders, loss of appetite, etc.). The duration of this period usually does not exceed 2-4 days.

• the period of the height of the disease - the symptoms most characteristic for the infection are most pronounced. Duration - from several days to several weeks.

• recovery period (convalescence) - the duration of this period depending on the form of the disease, the severity of the course, the effectiveness of therapy and many other reasons. Recovery can be complete when all functions impaired by the disease are restored, or incomplete if residual effects persist.

Infectious diseases are widespread among the population. According to the WHO, 52 million people die every year in the world, 17 million of them from infectious diseases. Of the 130 million children born annually on the globe, approximately 13 million die before the age of 14, 9 million of them from infectious diseases. The social significance of some infectious diseases gives an idea of ​​the indicators of their accompanying mortality (Table 11). Among the causes of death in different countries, infectious diseases occupy 3-4th place after cardiovascular and oncological pathologies.

Table 11

The number of deaths of some infectious diseases in the world

(B.V. Cherkassky, 2003)

Prevention of infectious diseases - the most important state, social and medical problem, which deals with the special science of epidemiology.

Epidemiology (from the Greek. Epi - over, demos - people and logos - science) - a medical science that studies the causes and characteristics of the spread of diseases in society with a view to preventing them.
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