[jennyp]

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.

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.

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.

[CobaltBlue]

Which would tend to suggest that present-day Botswanans are descended from people who were never exposed to HIV, or that, for some unknown reason, Africans with resistance to HIV died out for other reasons.

I've seen the data on European genetic resistance to HIV before - as I recall, Africans do not carry the genetic immunity to HIV found in Europeans.

I can't make out why there is speculation that immunity to HIV is somehow related to immunity to smallpox other than coincidence that both exist in the same populations. Given that smallpox has been eradicated, how can we know unless smallpox somehow becomes endemic/pandemic again?

I suppose they could test tissue from people carrying what is believed to be genetic immunity to HIV to see if they are immune to cowpox - if that's been tried I haven't seen it.