Check your roots: If your ancestors survived the Plague, you may have inherited a gene that can resist HIV.

The CCR5 gene—which normally ushers HIV into the immune system—can in rare cases mutate and confuse the virus so it can’t pass through. While studying the genetic makeup of thousands of AIDS trial subjects and honing in on those with CCR5 mutations, National Cancer Institute (NCI) biologists found the gene primarily in Caucasians, but not in native Africans and East Asians. So, when was the last epidemic that wiped out a mess of white folks and could have spurred a fighter gene? “Seven hundred years ago…the Bubonic Plague,” said NCI molecular biologist Steve O’Brien. The 14th-century Plague annihilated about one-third of the Euro population. This, O’Brien said, explains why up to 15 percent of white people— but not Africans or East Asians—have the mutation. “It just hit me in the face,” he said. “It wasn’t an idea. It was an accumulation of data.”

Harvard Med School’s Jerome Groopman, MD, termed the gene-tracing discovery “historical speculation” and questioned CCR5’s significance. “HIV evolves in a clever way and can find portholes of entry other than CCR5,” he said, adding that some strains of HIV can infect people with the mutation.