We are all Russian nesting dolls: Our intestines house many bacteria, which house many viruses. These so-called bacteriophages are likely as important for our health as the bacteria they live in. Lisa Brown for NPR hide caption

itoggle caption Lisa Brown for NPR

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

itoggle caption Will Kincaid/AP

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

itoggle caption Fayez Nureldine/AFP/Getty Images

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

itoggle caption Julia Bartoli, Chantal Abergel/AP

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

itoggle caption Stefan Hyman/University of Leicester

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

itoggle caption courtesy of Chantal Abergel and Jean-Michel Claverie

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

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

itoggle caption iStockphoto.com

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

itoggle caption iStockphoto.com

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