AIDS and the black death: a new PBS documentary connects HIV and the plague through people like Steve Crohn, whose ancestors' immunity to the black death has made them resistant to HIV. (television).

Few people can say with certainty that their ancestors had immunity to the black death that devastated Europe in the Middle Ages. New Yorker Steve Crohn can. What's more, the genes he inherited from those fortunate forebears may have made him largely immune to HIV.

Featured on "Mystery of the Black Death," the October 30 episode of PBS's Secrets of the Dead series, Crohn, 55, is a deep-voiced, no-bullshit freelance editor--and a survivor from the generation of gay New Yorkers swept away by the earliest wave of AIDS in the 1980s. Crohn's lover at the time, Jerry Green, was the fifth person in the United States reported to have died from AIDS complications. All but a handful of the couple's friends also succumbed to the scourge.

Crohn says he spent the 1980s waiting for his own demise. It never came. tie repeatedly tested HIV-negative.

He eventually learned that he has a genetic mutation called Delta 32--a "defective" genotype that prevents his being infected by HIV-1, the most common strain of HIV. It's not a happenstance that Crohn takes lightly. "This has a spiritual meaning to me for a couple of reasons," he reflects. "I hate people saying `I feel blessed'--I think that sounds so narcissistic."

Indeed, Crohn's participation in almost a decade of research into his genetic makeup--which may help others who do not have the Delta 32 mutation to battle HIV--has given him a profound sense of meaning. "It makes one feel like, yeah, you've been able to make a contribution," he says. "In some very small way your life has given something back. And this may affect the welfare and well-being of millions of people."

The Secrets of the Dead episode begins not with HIV but by investigating why so many residents of Eyam, England, survived the black death when it hit the remote village in 1665. Research by geneticist Stephen O'Brien has traced Crohn's Delta 32 mutation back hundreds of years to towns like Eyam, where the defensive genetic mutation held off the black death in much the same way it now protects Crohn from HIV. Today, about 10% of people of European heritage are estimated to be "Delta 32-heterozygous," which means they received the mutation from one parent and may have increased resistance to HIV. About 1% of that population, like Crohn, are Delta 32-homozygous, having inherited the genetic mutation from both parents.

Crohn learned about Delta 32 after he was enrolled in a 1994 Aaron Diamond AIDS Research Center study of a dozen HIV-exposed individuals who remained seronegative over long periods of time. When Scottish researcher Bill Paxton mixed Crohn's CD4 immunity cells with HIV in the laboratory, "we could not establish an infection," Paxton recalls, "even with a high dose of HIV. This was something we had never seen before and was extremely provocative and exciting."

Researchers eventually learned that people with the Delta 32 mutation lack a functional gene coding for producing a receptor called CCR5--one of two chemical "locks" on the surface of CD4 cells that HIV can penetrate. The discovery has helped fuel research into drugs that would similarly block infection--although Paxton points out that HIV's ability to mutate may render many such drugs ineffective over time.

Crohn himself has long relied not on his theoretical immunity to avoid infection but on safer sex. "[Even if] there's another strain out there," he says, "I know I'm not being that exposed."