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

Even in the remote Faroe Islands, some children have high levels of perfluorinated compounds in their blood. The chemicals may interfere with the immune system. Stig Nygaard/Flickr hide caption

itoggle caption Stig Nygaard/Flickr

Michele Pereira, who questions the schedule of mandatory vaccines, sat with her daughters, Evangeline, 6, right, and Genevieve, 2, at a park in Ashland, Ore., earlier this month. Jeff Barnard/AP hide caption

itoggle caption Jeff Barnard/AP

What's in this vial changes from year to year, but the process of creating the flu vaccine remains pretty constant.

Justin Sullivan/Getty Images hide caption

itoggle caption Justin Sullivan/Getty Images