According to research done by Dr. Stephen O'Brien, a mutated gene known as delta 32, which is found in Black Death survivor descendants, stops HIV in its tracks. The discovery was made by combining the sciences of DNA and genealogy.
In order to be immune, both parents must have the delta 32 gene. From the Article: "In 1996, research showed that delta 32 prevents HIV from entering human cells and infecting the body." O'Brien thought this principle could be applied to the plague bacteria, which affects the body in a similar manner. To determine whether plague survivors in one British town may have carried delta 32, O'Brien decided to test the DNA of their modern-day descendents.
Dr. O'Brien then worked with historian John Clifford, who used traditional genealogy techniques to identify descendants of plague survivors. Clifford focused on the small, central-England village of Eyam. In September 1665, George Viccars, a tailor in Eyam, received a parcel of cloth ridden with plague-infected fleas from London. Four days later, Viccars died. By the end of the month, five more villagers had succumbed to the plague. The panicked town turned to their rector, William Mompesson, who persuaded them to quarantine the entire village to prevent the bacterium from spreading throughout the region. It seemed like suicide.
A year later, the first outsiders ventured into Eyam, expecting a ghost town. Yet, miraculously, half the town had survived, a much higher survival rate than that found in other villages. Apparently, many of the villagers had some form of immunity, which now turns out to be something that one can study in order to learn more about prevention of HIV.
John Clifford began by examining the Eyam church register, noting everyone who was alive in 1665, the year the plague came to Eyam. He searched for evidence of life through the year 1725 -- marriages, baptisms, burials that took place years after the plague had left the village. Deleting the names of those lost during the plague period, he was able to determine who the survivors were. Dr. Stephen O'Brien then obtained DNA samples from the descendants of those survivors.
You can read more about this fascinating genealogy and science study at http://www.pbs.org/wnet/secrets/case_plague