Dr. Paul J. Pockros, a liver specialist at Scripps Green Hospital in San Diego, talks with hepatitis C patient Loretta Roberts in Jan. 2011. Lenny Ignelzi/AP hide caption

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Lenny Ignelzi/AP

An electron micrograph of human norovirus. Charles D. Humphrey/CDC Public Health Image Library ID 10708 hide caption

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Charles D. Humphrey/CDC Public Health Image Library ID 10708

Noah Stewart shelters in the closet just 15 minutes before an April 2011 tornado demolished his house. Wearing the helmet may have saved his life, one doctor says. Courtesy of the Stewart family hide caption

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Courtesy of the Stewart family

Ramon Maldonado-Cardenas grimaces as he gets a flu shot from pharmacy student Khoa Truong during a health fair in Sacramento, Calif., last October. Rich Pedroncelli/AP hide caption

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Rich Pedroncelli/AP