The cause of infectious diseases is the pathogen (microorganism). It forms a complex biological interaction with the body, which eventually leads to an infectious process, and subsequently to an infectious disease. Usually, each infectious disease has its own pathogen. However, there are exceptions when one disease may have several pathogens, for example, sepsis. Or, on the contrary, when one pathogen (streptococcus) causes several types of diseases (sore throat, scarlet fever, erysipelas). Every year, new pathogens of infectious diseases are discovered. Many pathogens of infectious diseases are visible under a conventional microscope, but there are also those that can be seen only when magnified thousands of times through an electron microscope. But people do not always get sick when they come into contact with pathogens of infectious diseases. This may be due to the innate or acquired immunity of a number of people to pathogenic microorganisms. Continuous compliance with infection prevention measures is essential in protecting against infectious diseases. The human body has protective barriers of the body that prevent the penetration of pathogenic microbes; these include: dry, clean, healthy skin, hydrochloric acid and stomach enzymes, white blood cells in the blood (white blood cells) that capture and destroy pathogenic microbes. The greatest effectiveness of such barriers is achieved in a hardened, healthy human body. The main causative agents of infectious diseases are protozoa, bacteria, spirochetes, rickettsias, chlamydia, mycoplasmas, viruses, etc. The largest part of infectious diseases is caused by bacteria and viruses. Protozoa are single-celled creatures capable of performing various functions characteristic of individual tissues and organs of more highly developed organisms. Bacteria are single-celled microorganisms of spherical (cocci), cylindrical (rods) or spiral (spirilli) shape. Spirochetes are mobile microorganisms of a filamentous or spiral shape.

Rickettsia, chlamydia are intracellular microorganisms-parasites that occupy an intermediate position between bacteria and viruses. Mycoplasmas are microorganisms that do not have a cell wall, but parasitize outside the cells. Viruses are microscopic non-cellular forms of life that are able to penetrate into certain living cells and multiply in them.

The human body has a natural defense – these are antibodies (special proteins produced by the immune system) that interact with the corresponding virus and thereby effectively prevent the course of the disease.

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