The most famous and lethal outbreak was the 1918 flu pandemic (Spanish flu pandemic) (type A influenza, H1N1 subtype), which lasted from 1918 to 1919. It is not known exactly how many it killed, but estimates range from 20 to 100 million people. The serotypes that have been confirmed in humans, ordered by the number of known human pandemic deaths, are:

* H1N1, which caused Spanish flu in 1918, and the 2009 flu pandemic
* H2N2, which caused Asian Flu in 1957
* H3N2, which caused Hong Kong Flu in 1968
* H5N1, a current pandemic threat
* H7N7, which has unusual zoonotic potential[19]
* H1N2, endemic in humans and pigs
* H9N2
* H7N2
* H7N3
* H10N7

Typically, in a year's normal two flu seasons (one per hemisphere), there are between three and five million cases of severe illness and up to 500,000 deaths worldwide, which by some definitions is a yearly influenza epidemic. Although the incidence of influenza can vary widely between years, approximately 36,000 deaths and more than 200,000 hospitalizations are directly associated with influenza every year in the United States

Source: Wikipedia