 In addition to these annual epidemics , Influenza A virus strains caused three global pandemics during the 20th century : the Spanish flu in 1918 , Asian flu in 1957 , and Hong Kong flu in 1968 – 69 . These virus strains had undergone major genetic changes for which the population did not possess significant immunity . Recent genetic analysis has revealed that three @-@ quarters , or six out of the eight genetic segments , of the 2009 flu pandemic strain arose from the North American swine flu strains circulating since 1998 , when a new strain was first identified on a factory farm in North Carolina , and which was the first @-@ ever reported triple @-@ hybrid flu virus . 
