Preventing HIV transmission is not easy, even though most people know how the virus is spread and are aware that condoms can offer protection (see “Can AIDS Be Cured?”). An HIV vaccine remains elusive: although a recent trial suggested some success in low-risk populations, the level of protection was not high enough, or certain enough, to justify introducing the vaccine. The first generation of topical microbicides, designed to allow women to protect themselves if their partners refuse to use condoms, was not effective in clinical trials. Male circumcision has been the only clear success: three trials in Africa showed that it partially protected heterosexual men. We still need new protective strategies for women and men who have sex with men, the people most affected by the pandemic.

Anti-HIV drugs are known to help prevent transmission from infected mothers to infants, which has become rare in parts of the world where the drugs are widely available. Postexposure prophylaxis is recommended by the Centers for Disease Control: a 28-day course of antiretroviral medication after exposure is thought to prevent transmission about 80 percent of the time. But this treatment can’t be administered unless people recognize when they have been exposed.

Targeting preventive treatment even before exposure is one solution. Clinical trials being conducted in the United States, Africa, Asia, and Latin America are evaluating whether daily oral use of antiretroviral agents in high-risk groups can help prevent transmission. The drugs selected for evaluation are generally well tolerated, last a long time in the body (allowing for once-a-day dosing and some forgiveness for missed doses), and have shown protection in animal models.

While preëxposure prophylactic treatment seems costly, high-quality generic formulations are widely available at low cost. Another possible objection is that being able to take a pill a day for HIV prevention might cause people to stop using condoms or to have sex with more partners. But I believe it could make prevention services attractive to people who would otherwise deny their risk. Perhaps by serving as a daily reminder of that risk, it could even inspire people to take more steps to protect themselves.

The global spread of HIV will be stopped only if people demand highly effective prevention tools and if those tools become available. History shows that neither will happen easily. Arming people in their struggle against HIV can only help.

Robert Grant is an investigator at the gladstone Institutes at the University of California, San Francisco.