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Swine influenza (SI) is a respiratory disease of pigs caused by type A influenza that regularly causes pandemics. SI viruses do not normally infect humans; however, human infections with SI do occur, and cases of human-to-human spread of swine flu viruses have been documented. Swine influenza also called as swine flu, hog flu, and pig flu that refers to influenza is caused by those strains of influenza virus, called SI virus (SIV), that usually infect pigs endemically. As of 2009, these strains are all found in influenza C virus and subtypes of influenza A virus known as H1N1, H1N2, H3N1, H3N2, and H2N3. The viruses are 80-120 nm in diameter. The transmission of SIV from pigs to humans is not common and does not always cause human influenza, often only resulting in the production of antibodies in the blood. The meat of the animal poses no risk of transmitting the virus when properly cooked. If the transmission does cause human influenza, it is called zoonotic swine flu. People who work with pigs, especially people with intense exposures, are at an increased risk of catching swine flu. In the mid-20th century, the identification of influenza subtypes became possible; this allowed accurate diagnosis of transmission to humans. Since then, 50 confirmed transmissions have been recorded; rarely these strains of swine flu can pass from human to human. In humans, the symptoms of swine flu are similar to those of influenza and of influenza-like illness, namely, chills, fever, sore throat, muscle pains, severe headache, coughing, weakness, and general discomfort. Influenza A is a single-stranded RNA virus with eight different segments. When two viruses co-infect the same cell, new viruses can be produced that contain segments from both parental strains. [ABSTRACT FROM AUTHOR]