A prostitute in Johannesburg waits for a client on a street corner. An estimated two-thirds of sex workers in South Africa are HIV positive. Yoav Lemmer/AFP/Getty Images hide caption

itoggle caption Yoav Lemmer/AFP/Getty Images

A nurse takes a blood sample from Nkosi Minenhle, 15, in a mobile clinic set up to test students for HIV at Madwaleni High School in KwaZulu Natal, South Africa. Stephane de Sakutin/AFP/Getty Images hide caption

itoggle caption Stephane de Sakutin/AFP/Getty Images

So cute, but not cuddly. The Egyptian tomb bat, Taphozous perforatus, is a likely carrier of the Middle East respiratory syndrome virus, or MERS. Courtesy of Jonathan H. Epstein/EcoHealth Alliance hide caption

itoggle caption Courtesy of Jonathan H. Epstein/EcoHealth Alliance

Somali women and children wait to get medicine in July 2008, from a clinic run by Doctors Without Borders about 20 miles south of Mogadishu. AFP/AFP/Getty Images hide caption

itoggle caption AFP/AFP/Getty Images

A dromedary camel waits for a tourist to hop on its back in Petra, Jordan. The country has recorded two cases of the Middle East respiratory syndrome. Chris Jackson/Getty Images hide caption

itoggle caption Chris Jackson/Getty Images

A red blood cell infected with malaria parasites (blue) sits next to normal cells (red). NIAID/Flickr.com hide caption

itoggle caption NIAID/Flickr.com

Some scientists think new types of bird flus should arise only in chickens, not in labs. Here a worker collects poultry on a farm in Kathmandu, Nepal, where the H5N1 virus was infecting animals in October 2011. Prakas Mathema/AFP/Getty Images hide caption

itoggle caption Prakas Mathema/AFP/Getty Images

A man who just recovered from the H7N9 bird flu leaves a hospital in Bozhou, China, in April. Since early May, the number of new H7N9 cases has dramatically declined. AFP/Getty Images hide caption

itoggle caption AFP/Getty Images

A police officer guards Cambodia's famed temple of Angkor Wat. The powerful city-state collapsed in 1431 after suffering through two decades of droughts. Heng Sinith/AP hide caption

itoggle caption Heng Sinith/AP

Ali Maow Maalin said he avoided getting the smallpox vaccine as a young man because he was afraid of needles. He didn't want others to make the same mistake with polio. Courtesy of the World Health Organization hide caption

itoggle caption Courtesy of the World Health Organization

A mother and daughter walk home after attending a community meeting about eradicating female genital mutilation in the western Senegalese village of Diabougo. Finbarr O'Reilly/Reuters /Landov hide caption

itoggle caption Finbarr O'Reilly/Reuters /Landov