There weren't really any credible origin stories for HIV for years after the discovery of the virus and the syndrome it causes (AIDS). Eventually (more than a decade later), a reasonably good origin narrative evolved, but not until there were plenty of now-discredited origin theories: https://en.wikipedia.org/wiki/Discredited_HIV/AIDS_origins_t...
The transmission mechanism for one of them is "being in the same room with the other person", and the transmission mechanism for the other is "having gay sex with the other person". [1] Almost everyone had pre-existing behavioral immunity.
If you can only infect a tiny portion of the population, you can only cause a tiny number of deaths.
[1] This is not the primary transmission mechanism in Africa, but it's pretty much the exclusive mechanism as far as the Western history of AIDS goes, with the runner-up being "receive a blood transfusion from the other person".
The difference is that Westerners generally form small, closed sexual networks, while Africans participate in a gigantic sexual network including most of the local sexually active population.
The term of art here is the fairly awkward "multiple simultaneous sexual partners", but that concept isn't quite enough - a typical Westerner with multiple simultaneous sexual partners might be a man with a wife and a mistress, but when neither of them is cheating on him, that's still just a tiny, closed three-person network. But in Africa, the typical case is that most people have multiple simultaneous sexual partners, so even with the low transmission rate from heterosexual intercourse, the paths still exist for the virus to flow from one person in the society to (almost) anyone else.
(I don't think mettamage deserved to be downvoted. I specified gay sex for a reason, but the followup question "why not heterosexual sex?" is pretty obvious. It has a lower transmission rate, but that's a minor issue compared to the behavioral difference.)
If the average number of sexual partners in a community is 5, then a lot of links exist for the virus to flow through. But the links are of much "higher quality" if the partners are simultaneous, because in the serial case, a lot of paths through the graph are, when you look at them closer, made impossible by the chronology. If Marie sleeps with Jim, and then Jim sleeps with Agnes, and Agnes carries HIV, the virus can't flow from Agnes to Marie (along what appears to be a valid path in the sexual network) unless Jim goes back and sleeps with Marie again.
So it's not just about lifetime sexual partners, though it's certainly true that high average lifetime sexual partners contributes to spread.
is a man with many casual dates / FWBs / one night stands, not a man with two long term partners.
Note that one-night stands can never, by definition, be simultaneous with each other, though a succession of one-night stands can each be simultaneous with a long-term girlfriend.
What matters is the orifice/method (and whether giving or receiving). The CDC has data here [1]. Anal sex has a much, much higher rate of transmission (especially receptive), regardless of the gender of the participants. Oral sex has almost no transmission, regardless of the gender of the participants.
Just as a counterpoint to the "gay AIDS" narrative, AIDS is extremely low in the lesbian community because I don't think there are even any high-risk sexual activities available.
It's kind of humorous to me that lesbians are above the straight community on the "STDs as a judgement of moral purity" scale. Somebody tell the TV pastors that they're going to have to rework it.
1. https://www.cdc.gov/hiv/risk/estimates/riskbehaviors.html
> What matters is the orifice/method (and whether giving or receiving).
No, this isn't what matters. Anal sex has a higher rate of transmission. Receptive sex (of any variety) has a higher rate of transmission. But that isn't what matters.
What matters is the sexual behavior of the community as a whole. The higher transmission rate of anal sex is not enough to explain why heterosexual sex was so insignificant as a vector for AIDS -- heterosexual sex is safer, but there's much, much more of it!
But the only way to participate in the gay sexual culture of the period, which is what caused the problem, was by having gay sex.
Populations go up and up until they crash.
https://sfamjournals.onlinelibrary.wiley.com/doi/full/10.111...
Personally I believe that were the flu or cold to arise anew todya, we'd have seen something very similar (a pandemic), but presumably, due to the lower rate of connectedness, local populations evolved (both in the DNA sense, and in the immune learning sense) responses that dull the scale (and even then, some bad flu years are Really Bad, although nobody really freaked out about it). The plague killed what, 1 in 10 people in Europe?
The other important thing to realize is that COVID was and isn't and won't ever be a truly existential risk, medically speaking, and the existential risks came more from economic collapse.
(which I think that period is probably relevant in the comparison to COVID)