A human being exists in the ecosystem of immense complexity, which consists of millions of living creatures. Over 1,200 species, mainly microorganisms (bacteria, protoozoans, unicellar microfungi, viruses, and prions), as well as helminths, less frequently arthropods, are able to colonize the external surface of a human body (skin, mucosa), reproduce themselves, penetrate the internal environment of the body, cause pathological changes in organs and tissues accompanied by functional impairment and elicit defensive mechanisms of the body.
The penetration of some forms of living creatures, mainly microorganisms, into the highly-organized organisms is called «infection» (from Latin infectio - contamination). The infection causes the development of an infection process, which may either run without health deterioration, symptom-free, or be accompanied by health deterioration of various severity, i.e. take a form of an infectious disease. Infectious disease, broadly speaking, means any disease caused by the invasion of living parasites, mainly microorganisms.
Infectious diseases take an important place in human life. They are rated as mass diseases. Every person carries some various infectious diseases (infantile infections, ARDS, AEIs, helminthic invasions, etc.) during his/her lifetime. These diseases are the main cause of medical care encounters and temporary incapacity for work. The annual infectious and parasitic disease death rate is over 16 million people, which is equal to the death rate from cardiovascular diseases together with cancer. In contrast to cardiovascular and oncological diseases, infectious diseases affect predominantly children and people of reproductive age, which makes them a powerful demographic factor. Infectious diseases are the main cause of death among children under the age of five, especially in developing countries, being the main factor that determines the life span.