Two boys study together at a Chicago yeshiva in 2009. Public health officials say this type of close physical contact caused a mumps outbreak to spread throughout several orthodox Jewish communities in and around New York City. M. Spencer Green/AP hide caption

itoggle caption M. Spencer Green/AP

Health workers transport the polio vaccine by donkey in southeastern Pakistan. Asad Zaidi/Courtesy of UNICEF Pakistan. hide caption

itoggle caption Asad Zaidi/Courtesy of UNICEF Pakistan.

Nurse Fatima Guillen (left) gives 4-year-old Kimberly Magdeleno a whooping cough booster shot at a health clinic in Tacoma, Wash., in May. Ted S. Warren/AP hide caption

itoggle caption Ted S. Warren/AP

A health worker in the Domincan Republic sprays insecticide between houses to stop dengue fever outbreaks this month. Erika Santelices/AFP/Getty Images hide caption

itoggle caption Erika Santelices/AFP/Getty Images

Thousands of doses of cholera vaccine sit in a refrigerated trailer in a United Nations compound in Saint-Marc, Haiti, in March. After some delays, a vaccination project proved successful. John Poole/NPR hide caption

itoggle caption John Poole/NPR

Pharmacist Kristy Hennessee administers a vaccination against whooping cough in Pasadena, Calif., in 2010. Vaccinations are the most powerful weapon for slowing the epidemic, but there are growing concerns that the current vaccine doesn't last as long as expected. Robyn Beck/AFP/Getty Images hide caption

itoggle caption Robyn Beck/AFP/Getty Images

Vampire bats are common in Central and South America, where they feed on livestock and sometimes people. Michael & Patricia Fogden/Corbis hide caption

itoggle caption Michael & Patricia Fogden/Corbis

A lone pig roots through trash dumped over the side of a sewage canal that runs from the center of Port au Prince through Cite de Dieu. During the rainy season, the canal overflows its banks and fills nearby houses with sewage, which can carry cholera. John W. Poole /NPR hide caption

itoggle caption John W. Poole /NPR

A nurse in Washington administers the whooping cough vaccine to a child in May. In response to the epidemic, more than 82,000 adults have also received the vaccine this year. Ted S. Warren/AP hide caption

itoggle caption Ted S. Warren/AP