Welcome, then, are a trio of studies in the New England Journal of Medicine demonstrating that Truvada, an anti-retroviral drug already on the market, can prevent new HIV infections when taken daily. The findings, which were released in part last year and appear this week in final form, offer hope for men and women -- gay and straight -- who are at high risk for contracting HIV from their partners. "What we're looking at here is a new HIV prevention strategy, an approach that hadn't been tested before," said Jared Baeten, an infectious diseases specialist at the University of Washington's School of Public Health. "By having the medication already in their blood stream and in their cells, by the time they came into contact with virus, it would block the virus from taking hold. It would block them from getting infected."

Baeten's study followed 4,700 "serodiscordant" couples -- in which one member was HIV-positive and the other negative -- in Kenya and Uganda. Previous research had shown the efficacy of Truvada in reducing the risk of the disease among men who have sex with men, and Baeten's team hoped to expand the data to heterosexual partners. In addition to receiving a daily drug -- either Truvada or a placebo -- the HIV-negative subjects received STI testing, AIDS awareness counseling, and access to condoms. At the end of the trial, men and women in the Truvada arm of the study were 75 percent less likely to contract HIV than their untreated counterparts. (That figure, while impressive, is even conservative. It included every subject who was given Truvada, regardless of whether they remembered to take it.)

Later, Baeten went back and isolated just those participants who actually took the drug, as evidenced by their blood work. Their level of protection? Ninety percent.

Bottom line, says Baeten: "Compliance with this medication is the key to it working." Missed doses may be forgiving for a day or two, but physicians aren't certain, and consider the drug safe enough -- side effects include nausea and dizziness, and in some cases a loss in bone density -- to give to otherwise healthy patients on a daily basis.

"The challenge is identifying people who are at risk who will actually take the pills as prescribed," says Dr. Timothy Mastro, of FHI 360, in North Carolina. Mastro worked on a study involving 2,100 HIV-negative women in Tanzania, Kenya, and South Africa, which was closed down early after Truvada failed to protect women any better than the placebo. When researchers parsed the data, they discovered that fewer than 40 percent of the women were taking the pills given to them. The problem wasn't the drug -- it was the patients.

Mastro points out that even disappointing results offer important lessons for public health officials. In the Baeten study, which dealt with couples, the negative partner "knew that they were going to bed at night with an HIV-infected person, which was probably a great motivator to really take their pill." The women in his own study were, on average, ten years younger, not in stable relationships, and lived in rural fishing communities where HIV was rampant. They were less likely to know their partner's status, and more than ten percent had traded sex for money or gifts in the month before. Even by the end of the study, a full three-quarters of the women told researchers that they were at "no or low risk for HIV infection" despite reams of evidence to the contrary. "We have a lot more to learn about how women in these settings perceive and react to risk," Mastro says.