The H1N1 swine flu virus kills some people, while others don't get very sick at all. A genetic variation offers one clue. Centre For Infections/Health Pro/Science Photo Library/Getty Images hide caption

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Bruno Mbango Enyaka gets his flu shot at a community health center in Portland, Maine, on Jan. 7. Gabe Souza/Press Herald via Getty Images hide caption

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A rogues gallery of the viruses (left to right) that cause MERS, SARS, and influenza. Niaid; 3D4Medical; Niaid/Science Source hide caption

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Joseph Robert gets a flu shot at the Metropolitan Multi-Service Center in Houston during the Falls and Flu Prevention Day, sponsored by the National Council on Aging and Sanofi Pasteur, on Sept. 26, 2013. Aaron M. Sprecher/AP Images for National Council on Aging and Sanofi Pasteur hide caption

itoggle caption Aaron M. Sprecher/AP Images for National Council on Aging and Sanofi Pasteur

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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