A Pakistani man wheels Jamshid, an 8-year-old girl with polio, around the outskirts of the capital Islamabad last July. Behrouz Mehri/AFP/Getty Images hide caption

itoggle caption Behrouz Mehri/AFP/Getty Images

The Anopheles stephensi mosquito transmits the malarial parasite while dining on human blood. You can find this type of mosquito in Afghanistan, China, India, Thailand and the Middle East. CDC hide caption

itoggle caption CDC

Dr. Paul J. Pockros, a liver specialist at Scripps Green Hospital in San Diego, talks with hepatitis C patient Loretta Roberts in Jan. 2011. Lenny Ignelzi/AP hide caption

itoggle caption Lenny Ignelzi/AP

Kotzebue, Alaska, is a remote arctic community of some 3,000 people. Alaska public health official Dr. Michael Cooper says that when he worked here three years ago, he occasionally saw patients with classic symptoms of tuberculosis — but he failed to make the connection. J. Stephen Conn/flickr hide caption

itoggle caption J. Stephen Conn/flickr

An electron micrograph of human norovirus. Charles D. Humphrey/CDC Public Health Image Library ID 10708 hide caption

itoggle caption Charles D. Humphrey/CDC Public Health Image Library ID 10708

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

itoggle caption Aaron Tam/AFP/Getty Images

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