In host-parasite systems — specifically, disease systems — one member of the co-evolving pair preys on the other. In such a system, scientists expect the host to evolve increased resistance to the parasite. All things being equal, being resistant must always be better.
So how should scientists expect the disease organism to evolve? A common misconception is that selection will cause the disease organism to evolve in a way that's less harmful to its host. The rationale: Because the disease organism needs the host, it should be nice (i.e., less virulent) to it. (Virulence refers to the fitness decrease that results from infection with a particular disease. A disease that kills you right away has a high virulence; if it just gives you the sniffles, and you can still make it to the office, it has a low virulence.)
But that's not the way it works. To a disease organism, maximum fitness isn't just about surviving; it's also about spreading to other hosts. Therefore, evolution selects for whatever virulence level makes the disease more effectively transmitted. A disease will become highly virulent, for example, when high virulence increases its fitness even as it decreases its host's fitness.
The specific virulence that maximizes fitness varies from organism to organism, based on how the disease is transmitted. This point is key. No one "right" level of virulence exists. The common cold won't be more fit if it kills me before I can make it to school, but when I'm at school, it's more fit if it makes me cough on someone else.
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