Many parasites never see the light of day, but they must still adapt to the environment around them. For many years, parasites were thought to be somehow degenerate ­ not as complex and sophisticated as free-living organisms. But in fact, parasites are highly adapted. The environments they live in are among the most hostile in the world. Gut parasites, for instance, must be able to withstand acidic surroundings; blood parasites must fight off their host’s immune system.
Their environments are created by the genes of their hosts. And just as the external environment can change, so too can the world of a parasite. Some parasites are nimble in this regard: trypanosomes constantly change their external coats so that the immune system of their host is always a step behind, hunting something that is no longer there.
