In relatively small prophylactic doses, like those in Truvada, they can protect the healthy from getting infected. In triple-therapy treatment doses (“cocktails” of three drugs), they can render the infected virtually noninfectious.

Evidence of how well they work keeps piling up.

The C.D.C. acted because blood samples from the famous 2010 iPrEx study showed that gay men who took Truvada seven days a week reduced their chances of infection by 99 percent. A 2011 trial, HPTN 052, of heterosexual couples in which only one was infected, showed that when the infected partners took their triple-therapy cocktails regularly, they were 96 percent less likely to infect their lovers.

And in a new and startling development, preliminary results were released in March from a European study following 767 gay and straight couples who didn’t use condoms and whose blood tests confirmed that the infected partners were taking their drugs. The pairs have had sex an estimated 30,400 times since the study began two years ago, and H.I.V. was not transmitted between them even once.

Protection of 96 to 100 percent is better than even the best vaccines. But vaccines protect for life while pills must be taken daily.

Some women found that hard with the Pill, so the contraceptive market eventually developed long-lasting injections, implants, vaginal rings and “morning after” pills.

The H.I.V. field is already moving that way.

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.)