Pathogen is a collective term for microorganisms and parasites that can cause disease. Microorganisms account for the vast majority, including viruses, chlamydia, rickettsia, mycoplasma, bacteria, spirochetes and fungi; parasites mainly include protozoa and worms. Pathogens are parasitic organisms, and the natural hosts parasitized are animals, plants and humans. There are more than 400 kinds of microorganisms that can infect humans, and they are widely present in human mouth, nose, pharynx, digestive tract, urogenital tract and skin.
Everyone may be infected by more than 150 kinds of pathogens in their life. Under the condition of normal human immune function, they will not cause disease, and some are even beneficial to the human body. For example, the intestinal flora (E. coli, etc.) can synthesize multiple vitamins. The existence of these flora can also inhibit the reproduction of some more pathogenic bacteria, so these microorganisms are called normal microbiota (normal flora). But when the body's immunity is reduced, the balance between humans and microorganisms is When destroyed, the normal flora can also cause diseases, so they are also called conditional pathogens (conditional pathogens). Whether the body is attacked by a pathogen or not, on the one hand, is related to its own immunity, on the other hand, it also depends on the pathogenicity of the pathogen and the number of invasions. Generally, the greater the number, the greater the possibility of disease. In particular, pathogens with weaker pathogenicity require a larger amount to cause disease. A few microorganisms are quite pathogenic and can cause disease with light infections, such as plague, smallpox, and rabies.