Cattle feeding practices have been changed in an effort to halt the spread of mad cow disease. iStockphoto.com hide caption

itoggle caption iStockphoto.com

Chickens were killed in Hong Kong last December in an effort to halt the spread of the deadly H5N1 strain of bird flu. Aaron Tam/AFP/Getty Images hide caption

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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

Marlene Lucien controls the hose that fills people's plastic buckets on one busy street corner in Port-au-Prince, Haiti. John Poole/NPR hide caption

itoggle caption John Poole/NPR

A micrograph shows red blood cells infected by the malaria parasite Plasmodium falciparum. John C. Tan/AP hide caption

itoggle caption John C. Tan/AP

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

Health Department officials cull birds and put them in sacks after bird flu virus was detected in Bhubaneswar, India. Biswaranjan Rout/AP hide caption

itoggle caption Biswaranjan Rout/AP

The C. difficile bacteria causes infections that kill about 14,000 Americans each year, the CDC reports. CDC Public Health Image Library hide caption

itoggle caption CDC Public Health Image Library

An health official wearing protective gear culls a bird at a poultry farm after a naturally occurring bird flu virus was detected near Agartala, India, in January. Sushanta Das/AP hide caption

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Ramon Maldonado-Cardenas grimaces as he gets a flu shot from pharmacy student Khoa Truong during a health fair in Sacramento, Calif., last October. Rich Pedroncelli/AP hide caption

itoggle caption Rich Pedroncelli/AP

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