I'm sure you've heard about the very ancient pedigree of tuberculosis by now. Carl Zimmer has posted in depth on this topic already, so I'll just point you there in case you want something more than the free PDF. Too often people imagine evolution as being driven out environmental change. My recent post which pointed to climate change is in keeping with this tradition. But in reality coevolution with other species, in particular pathogens, is a crucial factor which doesn't get enough emphasis, even though the 'Red Queen' hypothesis is in the zeitgeist. Unfortunately, I think part of this is the tendency to imagine 'fitness' as something in relation to the elements around you, when what is really crucial is your reproductive rate in relation to conspecifics. This rate is strongly influenced by persistent infections (pathogens) as well as social interactions with other members of your species. The "environment" is outside, inside as well as the biases and emergent dynamics of the social group. Additionally, the parasite-hitchikers can give us phylogenetically relevant data points for palaeoanthropology, as tapeworms and lice have.