San Francisco Supervisor Scott Wiener (left) says he started taking a drug to prevent HIV infection earlier this year. Lisa Aliferis/KQED hide caption

itoggle caption Lisa Aliferis/KQED

Truvada has been around for a decade as a treatment for people who are already HIV-positive. In the last few years, it has also been shown to prevent new infections, and New York officials are embracing the pill as a way to prevent the spread of AIDS. Justin Sullivan/Getty Images hide caption

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Dr. Lisa Sterman held a Truvada pill at her office in San Francisco in 2012. She prescribed Truvada to patients at high risk for HIV infection even before the Food and Drug Administration approved the medicine explicitly for that purpose. Jeff Chiu/AP hide caption

itoggle caption Jeff Chiu/AP

Dr. Lisa Sterman holds Truvada pills at her office in San Francisco. The drug was recently approved by the Food and Drug Administration to prevent infection in people at high risk of infection with HIV. The pill, already used to treat people with HIV, also helps reduce the odds they will spread the virus. Jeff Chiu/AP hide caption

itoggle caption Jeff Chiu/AP

Sir Elton John speaks Monday at the 19th International AIDS Conference in Washington. Carolyn Kaster/AP hide caption

itoggle caption Carolyn Kaster/AP

Longtime AIDS activist Dr. Ashraf Grimwood says South Africa has made huge strides in confronting HIV. But he worries that giving anti-retroviral drugs to healthy people could have negative consequences in the long term. Jason Beaubien/NPR hide caption

itoggle caption Jason Beaubien/NPR

Researchers with HIV medication at a public research lab at the Oswaldo Cruz Foundation, or Fiocruz, in Rio de Janeiro. Jason Beaubien/NPR hide caption

itoggle caption Jason Beaubien/NPR

Kevin Kirk (left) and James Callahan have been together for more than five years. Recently they sat down and talked about whether Kevin, who is HIV-negative, might want to start taking Truvada. Richard Knox/NPR hide caption

itoggle caption Richard Knox/NPR

Dr. Lisa Sterman holds up a Truvada pill at her office in San Francisco in May. Even before the Food and Drug Administration's approval, Sterman had prescribed Truvada for about a dozen patients at high risk for developing AIDS. Jeff Chiu/AP hide caption

itoggle caption Jeff Chiu/AP