Eight-year-old Manish, who caught polio years ago, learns to walk with leg braces at a rehabilitation center in New Delhi on Thursday. Manish Swarup/AP hide caption

itoggle caption Manish Swarup/AP

The fatality rate in an Ebola outbreak ranges from 25 percent to 90 percent, depending on the particular strain of the virus involved. Cynthia Goldsmith/ Centers for Disease Control and Prevention hide caption

itoggle caption Cynthia Goldsmith/ Centers for Disease Control and Prevention

Oxana and Pavel Rucsineanu fell in love while living at a tuberculosis ward in Balti, Moldova. Jason Beaubien/NPR hide caption

itoggle caption Jason Beaubien/NPR

Sophia Jarvis, 4, of Berkeley, Calif., is one of the few children diagnosed with the polio-like disease, which left her arm paralyzed. She attended a press conference Monday at Stanford University with her dad, Jeff. Martha Mendoza/AP hide caption

itoggle caption Martha Mendoza/AP

Jockeys take their camels home after racing in Egypt's El Arish desert. The annual race draws competitors from around the Middle East, including Saudi Arabia, where camels carry the Middle East Respiratory Syndrome virus. Nasser Nouri/Xinhua /Landov hide caption

itoggle caption Nasser Nouri/Xinhua /Landov

New research finds a close connection between the flu that devastated the horse population in North America in the 1870s and the avian flu of that period. Hulton Archive/Getty Images hide caption

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Hannah Rood, 3, receives an H1N1 vaccine at a clinic in San Pablo, California, during the 2009 swine flu epidemic. Justin Sullivan/Getty Images hide caption

itoggle caption Justin Sullivan/Getty Images

Small but costly: Dozens of mosquito species carry West Nile virus in the U.S. Darren McCollester/Getty Images hide caption

itoggle caption Darren McCollester/Getty Images