http://www.cdc.gov/h1n1flu/key_facts.htm
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Centers for Disease Control and Prevention, U.S. Department of Health and Human Services
H1N1 Flu (Swine Flu) is a respiratory disease of pigs caused by type A influenza virus that regularly causes outbreaks of influenza in pigs. Occasionally, swine flu can also infect humans. This page offers information on swine flu, plus symptoms, diagnosis, and spread of the virus in pigs and humans.