Editing Human pathogen (section)

==Virulence==
[[Virulence]] (the tendency of a pathogen to cause damage to a host's fitness) evolves when that pathogen can spread from a diseased host, despite that host being very debilitated.  An example is the malaria parasite, which can spread from a person near death, by hitching a ride to a healthy person on a mosquito that has bitten the diseased person.  This is called [[horizontal transmission]] in contrast to [[vertical transmission]], which tends to evolve symbiosis (after a period of high morbidity and mortality in the population) by linking the pathogen's evolutionary success to the evolutionary success of the host organism.

[[Evolutionary medicine]] has found that under horizontal transmission, [[Optimal virulence|the host population might never develop tolerance to the pathogen]].