A blood test developed by Harvard researchers checks for evidence of past infection with more than a thousand strains of virus, from about 200 virus families. The swine flu virus shown here, A/CA/4/09, rarely infects humans. C. S. Goldsmith/CDC hide caption

itoggle caption C. S. Goldsmith/CDC

Augustine Goba (right) heads the laboratory at Kenema Government Hospital in Sierra Leone. He and colleagues analyzed the viral genetics in blood samples from 78 Ebola patients early in the epidemic. Stephen Gire/AP hide caption

itoggle caption Stephen Gire/AP

Biologist Rob Knight, co-founder of the American Gut Project, recently moved the project to the University of California, San Diego's School of Medicine. Casey A. Cass/University of Colorado hide caption

itoggle caption Casey A. Cass/University of Colorado

Ecologists found signs of Ebola in a Rousettus leschenaultii fruit bat. These bats are widespread across south Asia, from India to China. Kevin Olival/EcoHealth Alliance hide caption

itoggle caption Kevin Olival/EcoHealth Alliance

A woman protects her child's face in Managua, Nicaragua, as health workers fumigate for mosquitoes that carry chikungunya. The virus started spreading through Nicaragua and Mexico in the fall. Esteban Felix/AP hide caption

itoggle caption Esteban Felix/AP

The dengue virus has an icosahedral shape, similar to the pattern on a soccer ball. Antibodies stop the virus by binding to its surface. Laguna Design/Science Source hide caption

itoggle caption Laguna Design/Science Source

Stringy particles of Ebola virus (blue) bud from a chronically infected cell (yellow-green) in this colorized, scanning electron micrograph. NIAID/Science Source hide caption

itoggle caption NIAID/Science Source

Caribous doing their business in mountain ice have left a viral record hundreds of years old. Courtesy of Brian Moorman hide caption

itoggle caption Courtesy of Brian Moorman

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