Truvada can dramatically reduce the risk of HIV infection when taken as a preventative medicine — if taken every day. Studies are underway to determine if young people are likely to take the pill consistently. Justin Sullivan/Getty Images hide caption

itoggle caption Justin Sullivan/Getty Images

Stringy particles of Ebola virus (blue) bud from a chronically infected cell (yellow-green) in this colorized, scanning electron micrograph. NIAID/Science Source hide caption

itoggle caption NIAID/Science Source

Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, talks with Doctors Without Borders staff during a visit in August to an Ebola treatment center in Monrovia, Liberia. Tommy Trenchard for NPR hide caption

itoggle caption Tommy Trenchard for NPR

Ebola patient Amber Vinson arrived by ambulance at Emory University Hospital on Oct. 15. Now healthy, Vinson was discharged from the hospital Tuesday. Kevin C. Cox/Getty Images hide caption

itoggle caption Kevin C. Cox/Getty Images

Dallas nurse Nina Pham speaks at a press conference after she was confirmed free of Ebola and released from a National Institutes of Health facility on Friday. Alex Wong/Getty Images hide caption

itoggle caption Alex Wong/Getty Images

At a CDC training session for clinicians headed to West Africa, a medical worker practices sanitizing hands after drawing blood from a mannequin portraying an Ebola patient. Brynn Anderson/AP hide caption

itoggle caption Brynn Anderson/AP