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

Human immunodeficiency virus Type 1 inserts its genetic material into the DNA of human cells, turning them into little HIV factories. Eye of Science/Science Source hide caption

itoggle caption Eye of Science/Science Source

While not conclusive, the two cases are "quite promising," says Anthony Fauci, a longtime AIDS researcher who directs the National Institute of Allergy and Infectious Diseases. Alex Wong/Getty Images hide caption

itoggle caption Alex Wong/Getty Images

Ruben Bermudez stands in front of a sign that says in Spanish, "To love yourself is to protect yourself." He has struggled to remain eligible for AIDS drug assistance programs since he went on treatment four years ago. Jessica Camille Aguirre/NPR hide caption

itoggle caption Jessica Camille Aguirre/NPR

Health care workers in South Africa speak to residents during a door-to-door AIDS awareness campaign, part of a series of prevention efforts that has helped lower the country's HIV infection rate. Mujahid Safodien /Reuters /Landov hide caption

itoggle caption Mujahid Safodien /Reuters /Landov

HIV patient Darnell Hollie, 47, talks to her doctor Monica Gandhi (right) at San Francisco General Hospital. Her path from drug addict to model patient was "a lot of work, but if you want it, it's there for you," Hollie says. Richard Knox/NPR hide caption

itoggle caption Richard Knox/NPR