A health official culls chickens on a poultry farm in a village on the outskirts of Katmandu, Nepal. Chickens suspected of being infected with H5N1 bird flu were found in the area in October. Prakash Mathema/AFP/Getty Images hide caption

itoggle caption Prakash Mathema/AFP/Getty Images

Clostridium difficile bacteria produce a toxin that damages the intestine and causes severe diarrhea. Courtesy of David Goudling/Nature Genetics. hide caption

itoggle caption Courtesy of David Goudling/Nature Genetics.

A child is inoculated with the polio vaccine at a traffic checkpoint just outside Pakistan's capital, Islamabad. Roadside vaccinations help health workers reach children in mobile populations. Jackie Northam/NPR hide caption

itoggle caption Jackie Northam/NPR

United Nations Aids Executive Director Michel Sibide hugs Secretary of State Hillary Clinton after they they presented the a road map for stopping HIV around the world. Jim Watson/AFP/Getty Images hide caption

itoggle caption Jim Watson/AFP/Getty Images

An artistic model of a coronavirus infecting a cell. The family of viruses can cause mild infections, like the common cold, and severe pneumonia-like infections, such as SARS. Peter Kohama/MCT/Landov hide caption

itoggle caption Peter Kohama/MCT/Landov

A young man places an oral swab into a solution to complete an HIV test during a free screening event in Washington, D.C. Brendan Smialowski/Getty Images hide caption

itoggle caption Brendan Smialowski/Getty Images

Hospitals began testing blood for hepatitis in 1992, so anyone who received a blood transfusion before then is at an increased risk for contracting the disease. iStockphoto hide caption

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Last year, Tom Mather caught 15,000 deer ticks in the woods of southern Rhode Island. "People really need to become tick literate," the University of Rhode Island researcher says. Brian Mullen for NPR hide caption

itoggle caption Brian Mullen for NPR

Students paste red ribbons on a window to mark World AIDS Day in Nanjing, China, in 2006. Between 2007 and 2011, the number of newly diagnosed HIV cases in China has nearly quadrupled to 40,000. AP hide caption

itoggle caption AP

Lyme disease is spread by deer ticks like this one. A study finds that some people can be reinfected many times with the bacteria that cause the disease. Lauree Feldman/Getty Creative Images hide caption

itoggle caption Lauree Feldman/Getty Creative Images

Health workers take a blood sample from an infant to test for the malaria at a clinic along the border between Thailand and Myanmar. Ben de la Cruz/NPR hide caption

itoggle caption Ben de la Cruz/NPR

A health worker in Pakistan marks a child's finger with ink after giving him the polio vaccine. Jackie Northam/NPR hide caption

itoggle caption Jackie Northam/NPR

As white-tailed deer have returned to New England in the past century, they've brought with them tick-borne parasites that cause human diseases. marcinplaza /iStockphoto.com hide caption

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