A recent experiment showed that humans have developed an effective defense against an extinct retrovirus, called PtERV (pronounced "pea-terv"). Chimpanzees haven't. The experiment suggested that this lack of a defense against one retrovirus actually makes chimpanzees immune to the effects of another, HIV.
Yeah, that's evolution in action, all right. So successful that it protects us from stuff that went extinct *millions* of years ago. Go figure...the intelligent designer ain't so smart when it comes to humans, eh?