HIV being relatively intransmissible.

I don’t know the biodynamics of a virus with its case fatality rate and latency period being susceptible to airborne transmission, but if it had been…

I have often thought about the timing of HIV's emergence.

Retroviruses were discovered only a few years before the first AIDS cases. The molecular amplification technologies that form the basis of HIV testing were only developed in the 1970s. The first antivirals that worked against HIV had been developed to treat other retroviruses, just a few years before, too.

If it had emerged in the early 20th century, there would have been no tests for it and no treatments for it. It would have been impossible to control. (We don't do a very good job of it even with all the tools we have now.)