Influenza A (H1N1) is a highly contagious acute respiratory disease of pigs caused by one of several swine influenza A viruses. Morbidity is high and mortality is low (1-4%). The virus is transmitted in swine herds by aerosols, direct and indirect contact, and by pigs carrying the virus asymptomatically. In temperate regions, interspecies outbreaks can occur throughout the year, but are more frequent in fall and winter. Swine are routinely vaccinated against influenza A (H1N1) in many countries. The most common influenza A (H1N1) virus is the H1N1 subtype, but other subtypes (e.g., H1N2, H3N1, H3N2) also exist. Like infection with the H1N1 influenza A virus, pigs can also be infected with avian influenza viruses and human seasonal influenza viruses. It was initially thought that the H3N2 swine flu virus was transmitted from humans to pigs. Sometimes pigs can be infected with more than one virus type at a time, allowing the genes of these viruses to recombine or reassort. This can result in a single influenza virus containing genes from many sources, called a recombinant virus. Generally, influenza A (H1N1) viruses are species-specific and only infect pigs, but they do sometimes cross population barriers and cause disease in humans.