Two men from northwestern Missouri became ill after tick bites infected them with a previously unknown virus. iStockphoto.com hide caption

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A Beechcraft airplane sprays insecticide over Dallas early Monday morning to curb the spread of West Nile virus. LM Otero/AP hide caption

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People wait in line at the Durham County Health Department for the H1N1 flu vaccination in Durham, N.C., in November 2009. Gerry Broome/AP hide caption

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Gregg Allman and Natalie Cole perform at the Tune In to Hep C benefit concert at the Beacon Theatre in New York on July 27, 2011. Both singers have battled chronic hepatitis C. Rob Bennett/AP Images for TuneInToHepC.com hide caption

itoggle caption Rob Bennett/AP Images for TuneInToHepC.com

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

itoggle caption 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

itoggle caption Courtesy of the Stewart family