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 H.I.V., the virus that causes AIDS, 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 Dr. Bruce D. Walker, the director of the Ragon Institute of Massachusetts General Hospital, M.I.T. and Harvard.