A recent survey of 200 young gay men conducted by Perry N. Halkitis, a behavioral researcher at New York University, found that if a three-month Truvada shot existed, 79 percent of them would prefer it over daily pills. Two studies in monkeys have already tested the long-lasting injectable concept, and it worked.

About 5,000 women are now testing vaginal rings that release virus-killing dapivirine into the vaginal walls. And in a recent study, monkeys who got vaginal doses of a fast-acting new drug, raltegravir, as late as three hours after sex were protected. (Admittedly, not “morning after” protection, but moving toward it.)

The new drugs permit even grander ambitions. By treating enough inhabitants, the whole “viral load” of a city can be lowered. That protects everyone — just as cities used to slow down smallpox outbreaks by rapidly vaccinating thousands of inhabitants.