Influenza in swine is an acute viral infection of the respiratory tract in pigs caused by type A influenza viruses. The mortality rate is low in pigs and recovery usually occurs within 7–10 days. Swine-origin (or swine-lineage) influenza virus (SIV) infections also occur in poultry and humans, but interspecies transmission is considered a rare event. Three subtypes of influenza A virus have been found to circulate in pigs: A(H1N1), A(H1N2) and A(H3N2). They are all swine influenzas, i.e. they are well adapted to their swine hosts. They are different from human influenza viruses with the same nomenclature and can be distinguished from them antigenically.