For years, evidence has showed that having genital herpes (HSV-2) along with HIV raises people's HIV viral load and their risk of transmitting HIV to sex partners. Based on these tried and true facts, researchers hoped that suppressing herpes in coinfected people could lessen the risk they would transmit HIV. But two years ago, disappointing results began to emerge in a study testing that theory. Now final results are in, and it's official: When HIV-positive people keep their HSV-2 in check by taking acyclovir, they do not lower the chances they will transmit HIV.

In the large (more than 3,000 couples) study, acyclovir did reduce coinfected people's HIV viral load—and it helped suppress herpes outbreaks. But the lower viral load failed to translate into a lower likelihood that their partners would contract HIV.