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

Nurse Susan Peel gives a whooping cough vaccination to a high school student in Sacramento, Calif. The whooping cough vaccine given to babies and toddlers loses much of its effectiveness by the time people reach their teens and early adulthood. Rich Pedroncelli/AP hide caption

itoggle caption Rich Pedroncelli/AP

U.S. Secretary of Health and Human Services Kathleen Sebelius (center) talks to a health worker during a visit to Eliazar Germain hospital in Port-au-Prince, Haiti, on Monday. It's Sebelius' first visit to the country. Ramon Espinosa/AP hide caption

itoggle caption Ramon Espinosa/AP

Rice farmer Alexi Rochnel shows his blank cholera vaccination card. April is the beginning of Haiti's rainy season, which will likely intensify Haiti's cholera outbreak. John W. Poole/NPR hide caption

itoggle caption John W. Poole/NPR

A makeshift latrine hangs over the water at the edge of Cite de Dieu, a slum in Port-au-Prince, Haiti. John W. Poole / NPR hide caption

itoggle caption John W. Poole / NPR

Rice farmer Alexi Rochnel shows his blank cholera vaccination card. April is the beginning of Haiti's rainy season, which will likely intensify Haiti's cholera outbreak. John W. Poole / NPR hide caption

itoggle caption John W. Poole / NPR

A young girl bathes in an irrigation canal. The canal and nearby river are the primary sources of water for most people who live in the country around Saint-Marc, Haiti. John W. Poole/NPR hide caption

itoggle caption John W. Poole/NPR

Thousands of doses of cholera vaccine sit in a refrigerated trailer in a United Nations compound in Saint-Marc, Haiti. Vaccination was supposed to begin last week, but bureaucratic problems have delayed the start. April is the beginning of Haiti's rainy season, which will likely intensify Haiti's cholera outbreak. John Poole/NPR hide caption

itoggle caption John Poole/NPR

Nurse Susan Peel gives a whooping cough vaccination to a student at Inderkum High School in Sacramento, Calif., in 2011. Now it seems likely such shots will become routine for senior citizens, too. Rich Pedroncelli/AP hide caption

itoggle caption Rich Pedroncelli/AP