I'm going from memory, and I can't remember how many years ago I saw this on television, but I thought it had something to do with a genetic mutation that likely gained a foothold in the Western European population sometime around the plague of the Middle Ages. If I remember correctly, if a person carried one copy of the gene, they could get infected with HIV, but it wouldn't develop into AIDS. If a person carried two copies, they would not even get infected with HIV.

Anybody remember hearing something like this?