It's possible that around 14% of the population of Europe and the United States have at least a resistance, if not total immunity to HIV.
Apparently, because their ancestors survived the Black Plague.
A mutation that affects the protein structure of the cell membrane of immune cells, referred to as Delta 32, has been found to be present in significant numbers among both direct descendents of bubonic plague survivors, and among some gay men who received heavy exposure to the virus in the 1970's through unprotected sex with infected men who later died of AIDS, and yet failed to become infected themselves.
Tests in which the blood of these individuals is exposed to levels of HIV thousands of times higher than a typically infective dose have revealed that no matter what dosage of HIV they are exposed to, their cells are not penetrated by the virus. Apparently, this is due to the absence of a surface protein that bonded to by the protein coat of the HIV virus.
An individual may have either one or two copies of this mutation. It is suspected that individuals with one copy are the source of stories dating to the time of the Black Death that recall an individual being exposed to infection, acquiring symptoms of the disease, and then recovering. Their recovery was often attributed to external factors (one woman is said to have recovered because she drank bacon grease), but some scientists now suspect that genetic defense was the real answer.
Others, who are presumed to have had two copies of the mutation, failed to develop symptoms, despite constant exposure to the disease. Two anecdotes from a small village in England where studies where first carried out refer to a widow whose husband and six children all succumbed, but who never developed symptoms herself, and to a village grave digger who handled virtually every infectious corpse in the village without ever falling ill.
Back in the 20th Century, individuals with a single copy of the mutation have been observed to experience a longer delay in the onset of infection than those without it. Individuals with two copies of the mutation are those who seem to be totally immune to the infection.
Before you go out and go crazy barebacking the night away, sharing needles, and transfusing mysterious blood you find on the street, recall that global studies show that in the areas where the Black Death raged (and among their descendents), around 14% of the population has at least one copy of the mutation. In Asia and Africa, and among native North Americans (American Indians, silly, not you), the percentage is effectively zero, as the ancestors of the inhabitants of these areas didn't pass through the Bubonic Plague bottleneck. The percentage of this mutation is calculated to have been at its highest around the time of the Black Plague, and since then has been diluted.
Ultimately, it may be possible that this mutation may be exploited to 'inoculate' the uninfected, or even to treat the infected, through some form of gene therapy. Unfortunately, the therapeutic use of genetics remains in its infancy, and the mutation needs to be studied in greater depth. Strangely, while attention to AIDS in the media and the medical community has been intense (though arguably, particularly in the case of the crisis in Africa not as intense as you'd hope), there were almost no studies of why certain at-risk individuals weren't becoming infected- most clinical trials and studies right up until today have been on infected individuals.
The first studies of an apparently immune individual- a gay man named Steve Crohn- were carried out in the late 90's/early 00's by doctors in New York City. Crohn had been active in the gay scene in San Francisco right before HIV and AIDS were recognized. His lover was among the first individuals in the U.S to be recognized as having died of AIDS. In the decade following the discovery of AIDS, Crohn claims to have lost between 70 and 90 close friends and former lovers- all of whom engaged in the same practices as Crohn himself. Crohn was not infected, and volunteered for testing that showed that his blood seemed to be basically impervious to HIV. Testing has indicated that Crohn has two copies of the relevant mutation.
So some of us are immune to HIV. And the Bubonic Plague. And hopefully, we'll eventually know more. And in the meantime, don't take any stupid chances.
. Yes, PBS