Originally posted by Dartmoor Dave Evolution by natural selection happens because of changes to the environment a species inhabits, not because of short term disease outbreaks. And it isn't survival of the fittest. Individuals that happen to have random mutations that suit the changed environment survive and reproduce, but they're not "fitter" or superior in any way. Just randomly luckier.
In all honesty, it *could* happen, in the case of, say, a hypothetical disease that targeted exclusively patients of blood type AB for example, since that's only a genetic factor. Still, off the top of my head I cannot think of any sickness - be it the plague, the 1918 flu, this one, you name it - that had genetics as a relevant factor.
There is one major exception (malaria), but it comes in the form of two suicidal mutations involving defective blood cells, so if you don't happen to catch malaria congratulations, your body is a giant liability for no benefit whatsoever.