Type AH1N1Influenza is a highly contagious acute respiratory disease of pigs caused by one or more types of swine influenzaA6169_6191(1-4%). The virus spreads through aerosols, direct and indirect contact, and pigs that carry the virus but are asymptomatic. Swine outbreaks can occur throughout the year. Incidence increases during autumn and winter in temperate zones. Many countries routinely vaccinate pig herds against swine influenza.

The most common influenza virus isH1N1subtype, but other subtypes are also circulating in pigs(For exampleH1N2、H3N1、H3N2). In addition to swine influenza viruses, pigs can also be infected with avian influenza viruses and human seasonal influenza viruses. It is believed thatH3N2Swine viruses were originally transmitted from humans to pigs. Sometimes pigs can be infected with more than one type of virus at the same time, allowing the genes of these viruses to mix. This can produce influenza viruses that contain genes from several sources, known as "recombinant" viruses. While swine influenza viruses are usually species-specific and infect only pigs, they sometimes cross species lines and cause disease in humans.

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