There is a gene called Delta 32 (or something like that) that blocks entry of foreign objects into certain cells of your body. It was present in a village in England during the Black Death (gahh forgot the name), but apparently, people with the gene (whether they have one or two copies of it) are immune to/will recover from the plague and aids.
There was a show about this, and a man had this gene in America during the 1970's (he had HIV, but it never manifested into AIDS).