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.