I got the above date from a google search. So I'm assuming they began treating with AZT in '84 or a little later. People died from this disease prior to that. What's the answer for those deaths?
Supposedly, the first cases of immune suppression were drug-related, gays going hog-wild with poppers (amyl nitrates?), which I believe Duesberg would claim have been shown to cause immune suppression. Plus, I believe that the general level of disease floating around the gay community in that era gave them something to be susceptible to if their immune system was shot. There's a ton of information accessable through the URL I provided earlier (and those provided by others) and some pretty compelling reading as well.
Doc Savage may be an actual doctor, and his first post was an admirable, if terse overview, so maybe he could contribute more info as well.