PPT Slide
About 50 years ago, an evolutionary biologist named J.B.S. Haldane suggested that the interaction between parasite and host (or predator and prey) should resemble an evolutionary arms race:
First a parasite (or predator) evolves a trait that allows it to attack its host (or prey).
Next, natural selection favors host individuals that are able to defend themselves against the new trait.
As the frequency of resistant host individuals increases, there is natural selection for parasites with novel traits to subvert the host defenses.
This process continues as long as both species survive.
Recent data on Plasmodium, the cause of malaria, support this model.