This year's flu vaccine protects against a few different virus strains, including the H1N1 seen here. The fuzzy outer layer is made of proteins that allow the virus to attach to human cells. NIAID/Flickr hide caption

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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

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Which flu vaccine should you get? That may depend on your age and your general health. Joe Raedle/Getty Images hide caption

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Influenza covers it's shell with two types of accessories: the H spike, blue, and the N spike, red. Here the flu particle is sliced open to show its genetic material. Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases hide caption

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Kimberly Delp gives a flu shot to Carleen Matthews at the Homewood Senior Center in Pittsburgh, Pa., last September. Andrew Rush/AP hide caption

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Sonia Despiar, right, a nurse with Gouverneur Healthcare Services, injects Imelda Silva with flu vaccine on Friday, Jan. 11, 2013, in New York. At least 10 elderly people and two children in New York have died from the flu and hospitalizations are climbing as the illness hits every county in the state. Bebeto Matthews/AP hide caption

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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

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People line up at a Duane Reade pharmacy in New York behind a sign announcing the recent flu outbreak. Andrew Kelly/Reuters/Landov hide caption

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Dr. Tom Nary is the director of health services at Boston College. Tovia Smith/NPR hide caption

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Physician assistants Scott Fillman (left) and Andrew Hunadi get ready to see patients with flu symptoms, in a tent erected just outside the emergency entrance at the Lehigh Valley Hospital in Allentown, Pa. Matt Rourke/AP hide caption

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