Years ago, Michael Kerr penned a letter to his doctor, letting the physician know that he was doing OK, that the HIV medications Dr. Jerry Rabinowitz had nudged him to take were working, and thanking Rabinowitz for helping to save his life. But Kerr never sent the note, embarrassed by the grammatical errors and rambling, run-on sentences.

He had forgotten about the letter, lost somewhere in his desk drawer, until Sunday, when he learned that Rabinowitz was one of the 11 people who died in a synagogue in Pittsburgh after a man opened fire during Shabbat services in what is believed to be the deadliest attack on the Jewish community in US history.

"I am grappling with the guilt that I never really got to thank him like I wanted to," Kerr told BuzzFeed News. "It's because of him that everything is alright with me and I am upset that he will never know how things worked out for me."

Rabinowitz, a 66-year-old physician, was renowned and beloved in his community for his constant, compassionate care, loyalty, and friendship. But Kerr said the doctor was a particular hero to Pittsburgh's LGBT community at the height of the AIDS epidemic.

"It was the mid-’80s and we had to figure out where we could go," Kerr said. "We looked at people who were living and who they were going to and it was Jerry. He kept people alive."

At the time, Kerr was 26 and getting his master’s degree in public health at the University of Pittsburgh. He found out he was HIV-positive after his friend, a lab supervisor, snuck him a test as part of a study. And that's when he sought out Rabinowitz.

Kerr saw the doctor often for treatment between 1989 and around 2004, before Kerr moved to New York. During those 15 years, the two became friends, a sentiment shared by other former patients of Rabinowitz in notes and tributes sent to TribLive and posted online this weekend.

"He would hold my hand, without gloves, when I was feeling really bad," Kerr said. "I referred my boss's mother to see him, and when she was dying, he was there with the family. He always made house visits. That's just how he was."

The doctor was known for his effusive smile, taking calls at all hours of the night, and being "someone you could just talk to," Kerr said, adding that he knew of a few dozen other people who saw Rabinowitz for their HIV treatment.