This reminds me of a PBS documentary I saw several times a few years ago dealing with genes that seem to provide resistance against the HIV virus.
Actually, it was a mutated gene that researchers theorized helped people either recover from or prevented them from coming down with the plague several hundred years ago.
The reason they looked at this is because the plague acts in much the same way as the HIV virus does.
The researchers went to this small little town that was hit hard during the plague, and a little over half of the village died, but there were accounts that some people got sick with the plague, but recovered, and others, even when in constant direct contact with the virus, never got sick. The researchers suspected a mutation that some people had in their genes helped protect them against the virus.
They used the old records and tested all the people living in the town who could trace their families back to those days, and they found that a small percentage of the people living their now had the mutation, either one or two copies of it, and when the researchers took into account the intermarriage of people from other towns over the course of several centuries, they realized that after the village had survived the plague, that a large portion of the population must have had the same mutation.
I seem to remember that what prompted this little venture was a research scientist in the UK I think that was tired of asking questions as to why all these people were getting sick with AIDS and HIV, and decided on another approach, and asked why certain people directly exposed to the virus weren't getting sick at all, and this research scientist met with this one guy who had lost most of his friends and his lover to AIDS. He had unprotected sex many times with those people who were eventually diagnosed with AIDS, but this was at a time when no one knew what it was.
All his blood tests showed negative for any trace of the HIV virus. And in repeated labratory experiments in where samples of his blood, along with several hundred others, were exposed to high concentrations of HIV, including the various strands of HIV, his blood sample repeated showed that the virus couldn't infect his blood.
They did a test on his DNA and found 2 copies of this same mutation in his DNA.
The guy is still alive and healthy, and even though through years of tests showing negative for HIV, he still gets himself tested on a regular basis, and takes care of his health.
I wish I had a link to the whole documentary.