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Influenza has been known to cause epidemics and pandemics for centuries.
Epidemics occur every two or three years, and more recently influenza epidemics
tend to occur every winter. The term pandemic applies to the occurrence of
epidemics throughout the world. In an epidemic the incidence of clinical
influenza may be as high as 40%. In the United States, the number of
influenza-associated deaths has varied from 20,000 to 40,000 per year. During
the 1918-1919 pandemic, more than 20 million people throughout the world died
from influenza. |
