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.

Green was one of the first people to die of the disease that became known as AIDS. In the ensuing years, scores of 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.

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 Dr. Bruce Walker, director of the Ragon Institute of Massachusetts General Hospital, MIT and Harvard.

Crohn died on Aug. 23 in New York City at 66.

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. Crohn had first come to the attention of Dr. Bill Paxton, then a scientist at the Aaron Diamond AIDS Research Center in New York. Paxton had been looking for gay men who seemed resistant to infection. Working with Dr. David Ho, now the chief executive of the Diamond Center, Paxton exposed 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 Crohn’s CD4 cells were flawed.

The genetic anomaly — the delta 32 mutation — which produces the flawed receptor, is found in less than 1 percent of the population.

“My brother saw all his friends around him dying, and he didn’t die,” his sister Amy Crohn Santagata said. “He went through a tremendous amount of survivor guilt about that and said to himself, ‘There’s got to be a reason.’”

“He was quite extraordinary, and then also quite ordinary,” she said.