Georgetown's Robert Clark says it's very difficult to say precisely how many experiments have been spoiled by contaminated cell lines. Phil Humnicky/Courtesy of Georgetown University hide caption

itoggle caption Phil Humnicky/Courtesy of Georgetown University

A turkey vulture makes quick work of a dead rabbit at Martin Luther King Jr. Regional Shoreline park in Oakland, Calif. Sebastian Kennerknecht/Minden/Corbis hide caption

itoggle caption Sebastian Kennerknecht/Minden/Corbis

Magnified 25,000 times, this digitally colorized scanning electron micrograph shows Ebola virus particles (green) budding from an infected cell (blue). CDC/NIAD hide caption

itoggle caption CDC/NIAD

This experimental Ebola vaccine, developed by the U.S. government, is just one of several undergoing small-scale, preliminary testing. University of Maryland School of Medicine/AP hide caption

itoggle caption University of Maryland School of Medicine/AP