A health official culls chickens on a poultry farm in a village on the outskirts of Katmandu, Nepal. Chickens suspected of being infected with H5N1 bird flu were found in the area in October.
Prakash Mathema/AFP/Getty Images
A child is inoculated with the polio vaccine at a traffic checkpoint just outside Pakistan's capital, Islamabad. Roadside vaccinations help health workers reach children in mobile populations.
United Nations Aids Executive Director Michel Sibide hugs Secretary of State Hillary Clinton after they they presented the a road map for stopping HIV around the world.
Jim Watson/AFP/Getty Images
An artistic model of a coronavirus infecting a cell. The family of viruses can cause mild infections, like the common cold, and severe pneumonia-like infections, such as SARS.
Last year, Tom Mather caught 15,000 deer ticks in the woods of southern Rhode Island. "People really need to become tick literate," the University of Rhode Island researcher says.
Brian Mullen for NPR
Students paste red ribbons on a window to mark World AIDS Day in Nanjing, China, in 2006. Between 2007 and 2011, the number of newly diagnosed HIV cases in China has nearly quadrupled to 40,000.
Lyme disease is spread by deer ticks like this one. A study finds that some people can be reinfected many times with the bacteria that cause the disease.
Lauree Feldman/Getty Creative Images