His boyfriend was dying of a disease without a name.
Beginning in 1978, Stephen Crohn cared for Jerry Green, a handsome gymnast, as he lost 30 pounds, went blind and was ravaged by the kinds of infections that rarely harmed otherwise healthy people.
Mr. Green was one of the first people to die of the disease that became known as AIDS. In the ensuing years, scores of Mr. Crohn's friends died of it. He had taken no special precautions, and he had been as sexually active as his friends.
But he never got sick.
Mr. Crohn's resistance helped lead to a deeper understanding of HIV, the virus that causes AIDS, and treatments, simply by staying alive and working with doctors to help figure out why he was.
"What he contributed to medical knowledge is really quite extraordinary," said Bruce D. Walker, director of the Ragon Institute of Massachusetts General Hospital, MIT and Harvard.
Mr. Crohn died on Aug. 23 in New York City at 66. The cause was suicide, his sister Amy Crohn Santagata said Friday in confirming the death, which was not announced at the time.
Mr. Crohn's immune system and its quirks earned him unsought renown.
In 1996, the British newspaper The Independent called him "The Man Who Can't Catch AIDS," and he told his story in documentary films and newspaper interviews around the world. Mr. Crohn had first come to the attention of Bill Paxton, then a scientist at the Aaron Diamond AIDS Research Center in New York.
Dr. Paxton had been looking for gay men who seemed resistant to infection. Working with David Ho, now the chief executive of the Diamond Center, Dr. Paxton exposed Mr. Crohn's cells, and those of another promising volunteer, to HIV.
"I couldn't infect the CD4 cells," he said in a telephone interview. "I'd never seen that before."
The CD4 white blood cells, which HIV normally penetrates to start the process of disease, locked out the virus. Even at HIV concentrations thousands of times greater than would be encountered outside a test tube, nothing happened.
Years later, researchers isolated the cause. HIV gets into cells by fitting into two receptors on CD4 cells. But thanks to a genetic defect, the second receptor on Mr. Crohn's CD4 cells were flawed. The malfunctioning receptor, CCR5, had no negative effect on his health and kept HIV from getting in.
The genetic anomaly, known as the delta 32 mutation, which produces the flawed receptor, is found in less than 1 percent of the population.
Stephen Lyon Crohn was born on Sept. 5, 1946, in Manhattan, to Richard Crohn (who also had the gene) and the former Janet Goren.
He was raised in Dumont, N.J., and educated at New York University, City College of New York and the Art Students League of New York. He lived in Saugerties, N.Y.
Mr. Crohn was an artist and worked as a freelance editor for Fodor's Travel. His paintings, mainly abstract works that evoked landscapes, were exhibited in New York, San Francisco and elsewhere.
Mr. Crohn was the great-nephew of Burrill B. Crohn, a leading gastroenterologist who first described the disease that carries his name.
Mr. Crohn felt he was carrying on "his family's tradition" by helping researchers, said Dr. Paxton, now a professor of infection and immunity at the University of Liverpool Institute of Infection and Global Health.
The research based on Mr. Crohn's immune system has led to advances in fighting HIV.
A drug that blocks the CCR5 receptor, maraviroc, is now used to keep infection from spreading in patients who have contracted the virus.
And in 2006, an AIDS patient in Berlin was effectively cured of the disease after receiving bone marrow transplants from a matching donor who had the delta 32 mutation.
"This is a classic case of medical science learning from patients," said Dr. Walker of the Ragon Institute. "Most of the immunology we know comes from studying other animal models," he said. "We need to study humans who have real diseases."
He explained, "You take the extreme examples and try to see how those people are different from the average person with the disease."
Dr. Paxton and Mr. Crohn remained friends. "He was the type of guy who walks into the room, and it lights up," he said.