Health workers in Nepal culled chickens and destroyed eggs following an outbreak of bird flu in Kathmandu in October 2012. Prakash Mathema/AFP/Getty Images hide caption

itoggle caption Prakash Mathema/AFP/Getty Images

A boy with multiple Guinea worms sits outside a containment center in northern Ghana, February 2007. Wes Pope/Chicago Tribune/MCT /Landov hide caption

itoggle caption Wes Pope/Chicago Tribune/MCT /Landov

When flu viruses (in red) accumulate an escape protein too quickly, they exit the cell nucleus (in blue) before they've made enough viral copies to spread the infection. Benjamin tenOever hide caption

itoggle caption Benjamin tenOever

A copper engraving from 1656 shows a plague doctor in Rome wearing a protective suit and a mask. Artwork by Paul Furst /Wikimedia.org hide caption

itoggle caption Artwork by Paul Furst /Wikimedia.org

Microbiologist Emma Allen-Vercoe invented the Robogut, a mechanical device that mimics conditions in the human colon. Courtesy of thestar.com hide caption

itoggle caption Courtesy of thestar.com

Registered nurse Michelle Newbury and physician assistant Scott Fillman see patients Thursday in a tent set up for people with flu symptoms, just outside the emergency entrance at the Lehigh Valley Hospital in Allentown, Pa. Matt Rourke/AP hide caption

itoggle caption Matt Rourke/AP

In the U.S., doctors no longer have the option of treating gonorrhea with a pill. Instead, they are advised to use an injectable antibiotic, which is still effective against the bacteria. iStockphoto.com hide caption

itoggle caption iStockphoto.com

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