Swine influenza (also called swine flu, hog flu, and pig flu) refers to influenza caused by those strains of influenza virus, called swine influenza virus (SIV), that usually infect pigs. Swine influenza is common in pigs in the mid western United States (and occasionally in other states), Mexico, Canada, South America, Europe (including the United Kingdom, Sweden, and Italy), Kenya, Mainland China, Taiwan, Japan and other parts of eastern Asia.

Transmission of swine influenza virus from pigs to humans is not common and properly cooked pork poses no risk of infection. When transmitted, the virus does not always cause human influenza and often the only sign of infection is the presence of antibodies in the blood, detectable only by laboratory tests. When transmission results in influenza in a human, it is called zoonotic swine flu. People who work with pigs, especially people with intense exposures, are at risk of catching swine flu.

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