"'m not exactly an evolutionist but the defect that caused many to survive the bubonic plague comes to mind. The plague got into cells via the same two protein receptors as HIV, and some people in western europe have a genetic defect which lacks one of those two protein receptors. therefore, it is impossible for those people to get the bubonic plague, and those people's descendents (the ones who inherited the defect) cannot get HIV, no matter how many times they are exposed."