http://www.cdc.gov/flu/keyfacts.htm
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Centers for Disease Control and Prevention, U.S. Department of Health and Human Services
The flu is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness, and at times can lead to death. The best way to prevent the flu is to get a flu vaccine each fall. This fact sheet provides key facts about influenza and the influenza vaccine, including how flu spreads, who should get vaccinated, and more.