The pandemic outbreak of influenza in 1918 claimed more than 20 million lives in just a few months.

Subsequent pandemic outbreaks in 1957 ("the Asian flu") and in 1968 ("the Hong Kong flu") were together responsible for more than 1.5 million deaths worldwide. The resulting productivity losses and medical expenses were estimated at 32 billion dollars.

Between 1972 and 1995, an exceptionally high mortality rate was reported in the United States during 19 of the last 23 recorded flu epidemics. Five of these epidemics caused 20,000 or more deaths; in six others, the death rate was over 40,000.

The influenza virus during the course of an epidemic may infect 5% to 20% of the entire population.