I'm leaning towards a "lifestyle" disease. The homosexuals in San Fran who engaged in passive anal sex with dozens of partners are likely to have picked up half the damn pathogens on the planet.
Similarly, what does a decade of IV drug use and needle sharing do to the immune system? If long term drug abuse results in immune system failure, you would expect to see this a decade or two after the 60's and 70's.
Perhaps HIV is the culprit. But no one has ever explained how it works and the original science was done in an extremely shabby manner by someone known for their dishonesty.