Modern HIV is 75 years old

Chimps seem to do fine carrying SIV, so presumably they've been infected for millenia - or longer. It now appears human infection is about 75 years old:

BBC NEWS | Health | HIV origin 'found in wild chimps'

The origin of HIV has been found in wild chimpanzees living in southern Cameroon, researchers report.

A virus called SIVcpz (Simian Immunodeficiency Virus from chimps) was thought to be the source, but had only been found in a few captive animals.

Now, an international team of scientists has identified a natural reservoir of SIVcpz in animals living in the wild.

It is thought that people hunting chimpanzees first contracted the virus - and that cases were first seen in Kinshasa, in the Democratic Republic of Congo - the nearest urban area - in 1930.

Humans may have contracted SIV before, but it never made the transition into an epidemic -- the victims would have died without passing on the disease. In 1930 it began the long ascent to its current pandemic status.

I was in medical school when HTLV-III was identified, I grew up in the pre-AIDS era. Things were different then.