# Pathogens --- **Pathogens** are a [[disease]]-causing [[microbe|microbes]]. The five major classes of pathogens are [[virus|viruses]], [[bacteria]], [[fungi]], [[protozoa]], and multicellular [[parasites]]. [[Virulence]] is a measurement of a how pathogenic—or likely to cause [[disease]]—a microbe is. **Opportunistic pathogens** are pathogens that normally we would be able to fight off quite easily, but can cause a secondary infection. ## Balanced Pathogenicity **Balanced pathogenicity** is a phenomenon where over time a pathogen becomes less virulent at the same time that the host becomes less susceptible to the pathogen. This benefits both the host and the pathogen itself. ### Benefits to Host The benefits of balanced pathogenicity to the host is quite straight forward: they become less susceptible to the pathogen and are less likely to suffer ill effects or die. ### Benefits to Pathogen The pathogen itself benefits from becoming less pathogenic, because it needs it's host to stay alive long enough to complete it's replication cycle. For example, when the [[Ebola virus disease|Ebola virus]] lives in it's natural home of a fruit bat or wild pig, it can just go on living it's best life, and the bat and pig show no symptoms. When it jumps to humans though, it's *too* deadly and it kills it's host *too* quickly, and doesn't have time to transfer to a new host. To quote CGP Grey: most germs don't want to kill you for the same reason that you don't want to burn down your house, *they live in you*. ___