Stephen Crohn, also known as "The Man Who Can’t Catch AIDS,” committed suicide last month at his home in New York at the age of 66. Crohn's sister, Amy, confirmed her brother’s death and that he faced years of despondency over the death of many of his friends due to the deadly virus.

"My brother saw all of his friends dying, and he didn't die," Amy told the New York Times. "He went through a tremendous amount of survivor guilt about that and said to himself, 'There's got to be a reason.'"

During the 1980s AIDS epidemic, Crohn watched a number of friends die from the virus, including his boyfriend Jerry Green. Grief-stricken, Crohn volunteered to help AIDS research under the care of Dr. Bill Paxton and Dr. David Ho of the Aaron Diamond AIDS Research Center in New York.

During their examination, Paxton and Ho noticed that Stephen’s CD4 white blood cells, where the HIV infection begins, were defective. Due to this genetic abnormality, doctors revealed a flawed CCR5 receptor that prevented HIV from entering Stephen’s body. Stephen’s condition, which affects less than one percent of the population, became known as the delta-32 mutation.

Stephen’s contribution allowed Paxton and Ho to lay the groundwork for how we understand the spread of HIV and subsequently AIDS. This research has led to better treatment options and a better chance for a future AIDS vaccine, the NY Daily News reported.

"What's hard is living with the continuous grief," Stephen said in a 1999 interview for the PBS documentary Surviving AIDS. "You kept losing people every year—six people, seven people. Last week, a friend of mine's obit. was in the paper. It's not easy, when you're losing friends and you're that young, and it goes on for such a long period of time. And the only thing you could compare it to would be to be in a war."