http://www.cdc.gov/flu/pandemic/keyfacts.htm
Centers for Disease Control and Prevention, U.S. Department of Health and Human Services
An influenza pandemic is a global outbreak of disease that occurs when a new influenza A virus appears or “emerges” in the human population, causes serious illness, and then spreads easily from person to person worldwide. This fact sheet discusses key facts about pandemic influenza , including appearance (emergence) of pandemic influenza viruses, influenza pandemics during the 20th century, vaccines to protect against pandemic influenza viruses, and antiviral medications to prevent and treat pandemic influenza.