Swine Flu

swine flu, also called swine influenza, hog flu, or pig flu, a respiratory disease of pigs that is caused by an influenza virus. The first flu virus isolated from pigs was influenza A H1N1 in 1930. This virus is a subtype of influenza that is named for the composition of the proteins hemagglutinin (H) and neuraminidase (N) that form its viral coat. Since the 1930s three other subtypes of flu viruses also have been isolated from pigs, including H1N2, H3N1, and H3N2. The emergence of H3N2 in pigs occurred in the late 1990s and is suspected of having been transmitted to pigs from humans. Although swine influenza viruses are similar to the influenza viruses that circulate among humans, swine viruses possess distinct antigens (molecules that induce an immune response).