There was a 2009 documentary called House of Numbers that talks about the HIV/AIDS situation. I don't think it's a good documentary; but there is some validity to what they discovered.

Back in the 80s/90s, the gay communities in San Francisco and New York had a lot of heath issues. Poppers, coke and meth even were all in heavy use. Many of these people would get penicillin shots for STDs, and repeated use of antibiotics within a single year clears the way for even worse infections. All of these things can obliterate your immune system over time.

There were probably multiple causes for GRID/AIDS. These weren't healthy people who had a sudden epidemic. These were people who were constantly being treated for STDs, with lots of health problems related to substance abuse. A percentage of GRID/AIDS deaths likely came from things other than HIV.

There are also "Long Term Non-progressives," people with HIV who chose not to get any treatment. There are a number of people who have HIV that will probably never develop AIDS, but it's unethical to pay some for trials and give them placebo treatments.

Herpes is another retrovirus that is likely present in the majority of humans (probably over 70% by some estimates; many who never present with cold sores). I wonder if some time in human pre-history, it killed offer a few million people, but now we all just live with it.