More on lice, from Carl Zimmer in his excellent blog, The Loom:
If pubic lice are not the sort of thing you want to be seen reading about, let me give you the opportunity to close your browser window right now. But if you’re at all curious about the secret that pubic lice have been keeping for over three million years, the tale of a mysterious liaison between our ancestors and the ancestors of gorillas–read on.
Many parasites tend to stick close to their hosts. A parasitic wasp may wander through forests and fields to find a caterpillar from a single species of butterfly in which it will lay its eggs. Blood flukes taste the water of their ponds for molecules from human skin. Wolbachia, a species of bacteria, never even has to leave its hosts, because it is passed down from mothers to their offspring. If a parasite sticks to its host for millions of years, their evolution may run on parallel tracks. As the host species splits in two, its parasite splits as well.
One of the best studied cases of parasites and hosts coevolving this way comes from pocket gophers and their lice…
More here.