Infectious disease
An infectious disease is a disease caused by the transmission of a micro-organism: virus, bacterium, parasite, fungus, protozoa The study of infectious agents belongs to medicine, microbiology, epidemiology and ecoepidemiology . In nature, infectious diseases develop in all living organisms. As long-lasting interactions, infectious diseases are part of the feedback loops that sustain the relative stability of ecosystems, with most pathogens co-evolving with their host for millions of years. Their mode of transmission is variable and depends on their reservoir and sometimes vectors. They are more or less contagious. For example, tetanus is a toxin caused by Clostridium tetani, a bacterium found in the earth. There is no human-to-human transmission; infection occurs when the bacteria enters the body through a soiled wound. A vaccine exists against this condition and is mandatory in France for all children of school age.