How do you know that HIV never "hit the human population" before? Remember, we're probably all "out of Africa" - where HIV is now pandemic. 40% infection rate among adults in Botswana.But then shouldn't the Botswanians & other Africans be more immune to AIDS? If AIDS is a very old recurring epidemic in Africa, then maybe they would have less immunity than the Northern Europeans, but still, to have 40% of your adult population infected & dying seems to say that there's no immunity there.After a plague there are two basic groups of survivors - those who never came in contact with it, and those who were immune to it.
For one thing, the people with the resistance aren't the ones who've been living there in the last 100,000 years.