Swine influenza is an acute, highly contagious, respiratory disease that results from infection with type A influenza virus. Field isolates of variable virulence exist, and clinical manifestation may be determined by secondary organisms. Pigs are the principal hosts of classic swine influenza virus. (Human infections have been reported, but porcine strains of influenza A do not appear to easily spread in the human population. However, deaths have occurred in immunocompromised people.) The disease in swine occurs commonly in the midwestern USA (and occasionally in other states), Mexico, Canada, South America, Europe (including the UK, Sweden, and Italy), Kenya, China, Japan, Taiwan, and other parts of eastern Asia.
The H1N1 form of swine flu is one of the descendants of the Spanish flu that caused a pandemic in humans in 1918