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

This human-scale lab rat cage is parked near a skate park in Denver, Colo., to make a point about the lack of science on marijuana. Richard Feldman Studio/Sukle Advertising and Design hide caption

itoggle caption Richard Feldman Studio/Sukle Advertising and Design

President Obama meets with Emory University doctors and health care workers during his visit Tuesday to the Centers for Disease Control and Prevention in Atlanta. Mandel Ngan/AFP/Getty Images hide caption

itoggle caption Mandel Ngan/AFP/Getty Images

Dr. Harold Varmus, a Nobel Prize winner, cancer biologist and director of the National Cancer Institute. Manuel Balce Ceneta/AP hide caption

itoggle caption Manuel Balce Ceneta/AP