Doctors at a hospital in Aleppo, Syria, treat a boy injured in what the government said was a chemical weapons attack on March 19. Syria's government and rebels accused each other of firing a rocket loaded with chemical agents outside of Aleppo. George Ourfalian/Reuters/Landov hide caption
The green dots are HIV virus particles on a human white blood cell. CDC hide caption
A baby receives a polio vaccine at the Medina Maternal Child Health center in Mogadishu, Somalia, on Wednesday. Somalia has one of the lowest immunization rates in the world. Ben Curtis/AP hide caption
People sit near pigeons at a park in Shanghai where more than 30 human cases of bird flu have been reported. AP hide caption
A health worker collects pigeons from a trap at People's Square in Shanghai, China, earlier this month. So far, workers have tested more than 48,000 animals for the H7N9 flu virus. ChinaFotoPress/Getty Images hide caption
A vendor weighs a live chicken at the Kowloon City Market in Hong Kong Friday. Health authorities there have stepped up the testing of live poultry from China to include a rapid test for the H7N9 bird virus. Lam Yik Fei/Getty Images hide caption
Renande Raphael, aged 16 months, is measured to check whether she is growing normally. She's part of a trial in Haiti to see if an extra daily snack of enriched peanut butter prevents stunting and malnutrition. Alex E. Proimos/via flickr hide caption
People sit near pigeons at a park in Shanghai Sunday. A new strain of bird flu has spread from eastern China to other provinces, with 13 deaths reported. AP hide caption
Workers prepare an H7N9 virus detection kit at the Center for Disease Control in Beijing on April 3. AFP/Getty Images hide caption
A child wears a mask near a closed section of a poultry market in Shanghai, where health workers detected the new bird flu, H7N9. Eugene Hoshiko/AP hide caption
Two prominent chins meet: Cary Grant and Ingrid Bergman kiss in the 1946 thriller Notorious. Hulton Archive/Getty Images hide caption
Dengue fever patients are treated in a hospital in Asuncion, Paraguay, in January. Norberto Duarte/AFP/Getty Images hide caption
A cockerel walks on a bridge in a residential area of Beijing. The Chinese are beginning to destroy thousands of birds in an effort to stamp out the presumed source of H7N9 infection. Wang Zhao/AFP/Getty Images hide caption
A child is immunized against polio at the health clinic in a farming village in northern Nigeria. The procedure involves pinching two drops of the vaccine into the child's mouth. For full protection, the child needs three doses, spaced out over time. David Gilkey/NPR hide caption