Infectious diseases are a group of diseases caused by the penetration of pathogenic (pathogenic) microorganisms, viruses and prions into the body. In order for a pathogenic microbe to cause an infectious disease, it must have virulence (toxicity; Latin virus - poison), that is, the ability to overcome the body's resistance and exhibit toxic effects. Some pathogenic agents poison the body with exotoxins secreted by them in the process of vital activity (tetanus, diphtheria), others release toxins (endotoxins) when their bodies are destroyed (cholera, typhoid fever). One of the features of infectious diseases is the presence of an incubation period, that is, the period from the moment of infection to the appearance of the first clinical signs. The duration of this period depends on the method of infection and the type of pathogen and can last from several hours to several years (the latter is rare). The place where microorganisms enter the body is called the entrance gate of the infection. Each type of disease has its own entrance gate, for example, Vibrio cholerae enters the body through the mouth and is not able to penetrate the skin.