Influenza A virus subtype H1N1 is the most common cause of flu in humans.  Some strains of H1N1 are endemic in humans, including the strain responsible for the 1918 flu pandemic which killed 50–100 million people worldwide.  Other strains are endemic in pigs and in birds.
A new H1N1 virus strain is responsible for the outbreak first detected in Mexico in March 2009. It appears to be a reassortment of 4 strains, 1 endemic in humans, 1 in birds and 2 in pigs. Although called swine flu due to it predominantly containing swine strains, the World Organisation for Animal Health has proposed the name North American influenza because this new strain in its entirety has not been found in swine.  This strain was formally declared as a global flu pandemic by WHO after an emergency meeting on 11th June 2009.

The "Spanish Flu" pandemic of 1918-1919 was the most deadly outbreak of influenza to occur in recent history. Worldwide, over 21 million people died of this influenza, many of them young and otherwise fit. It hit quickly and it hit hard -- it is said that a person could be feeling fine in the morning, feel a little poorly in the afternoon, and be dead by nightfall of the same day. It is thought to have originated somewhere in China. The first wave hit North America in the spring of 1918, brought in by soldiers returning home from the war. In the fall and winter of that year, it returned in a more devastating second wave that traversed the globe. The third wave, much less virulent than the second wave, hit in the spring of 1919.
Please only submit pages dealing with the 1918 Spanish Flu pandemic to this page. If your site deals with multiple pandemics or influenza in general, please submit to the main Influenza category instead.