France has banned imports of live pigs and other products from the U.S. to keep out a virus that has killed more than 4 million pigs in the U.S. Here, young pigs look out of a pen at a North Dakota hog farm in a 2005 file photo. Will Kincaid/AP hide caption

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Fearful of catching the MERS virus, workers wear masks during a soccer match on April 22 at King Fahad stadium in Riyadh. Fayez Nureldine/AFP/Getty Images hide caption

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This electron microscope image provided by researchers shows a section of a Pithovirus particle, dark outline, inside an infected Acanthamoeba castellanii cell. Julia Bartoli, Chantal Abergel/AP hide caption

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Clostridium difficile, a bacterium that causes severe diarrhea, can be difficult to treat with antibiotics. Stefan Hyman/University of Leicester hide caption

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Pandoraviruses were discovered lurking in the mud of Chile and Australia, half a world apart. courtesy of Chantal Abergel and Jean-Michel Claverie hide caption

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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

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Bats harbor many types of coronaviruses and were probably the original source of the new coronavirus that appeared in the Middle East. hide caption

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Two men from northwestern Missouri became ill after tick bites infected them with a previously unknown virus. hide caption

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

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