Dear Science,

A few sex experts insist that even if a cure for HIV were discovered tomorrow, we could never return to the days of unprotected sex with tons of random, anonymous strangers in clubs like the Mineshaft, because a new deadly STD would arise. I don't think this is so. There have been two truly deadly pandemic STDs: syphilis and HIV. Both are horrifying, but syphilis has evolved from a disease that killed in months to one that kills in decades, which makes sense. HIV might take the same path. So in the whole of human history, we have two deadly pandemic STDs, one that arose relatively recently and one that was around in South America for God knows how long before Europeans publicized it by dying from it in huge numbers. Even were we to assume that syphilis was brand-new in 1494, that would still be roughly 500 years between the emergence of these two. I assume that more STDs will evolve (it is a fine method of transmission), but why would it be likely that they'd be worse than gonorrhea or herpes?

Bedroom Evolutionist

Nothing in life is as motivating as a parasite, particularly a newly met parasite. The meeting can go down one of two broad paths. The first is an ever-escalating battle, with host and parasite devising newer and stronger defenses and attacks. The end of this interaction is extinction—for the parasite or for the parasite and host both. Rarely, if ever, do human parasites stick on this road for long. The human immune system is the fucking monster of the natural world. Think of it this way: Each of us alive today survived multiple massive pandemics. We're the progeny of the handful who lived through the plague and smallpox. Thank you, filthy, filthy human civilization!

The other path is for the parasite to become as benign to the host as possible—to be commensal. This is the hide-under-the-rug strategy. If the parasite can get in and do its thing with so little damage that the host doesn't even notice it, this is great for both.

Let's think about those dizzy moments you mentioned, when the human genetic pool met a new parasitic species. HIV is closely related to SIV in primates. In primates, SIV is commensal—most primates don't even notice having it. So HIV is simply a parasite that went down the first path; whereas SIV is a parasite that went down the second path.

Science cannot predict the future for you. A new STD, just by random chance, might start off rearing for a fight and start on the death-spiral path. Or it might creep in, silently commensal as it spreads in the population. Thanks to the (ongoing) HIV pandemic and the continuing consequences, many more people might try to sit on the sidelines and not discover firsthand which path the next parasite will take.

Collaboratively Yours,

Science

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