Swine influenza is an infection caused by any of several types of swine influenza viruses. Swine influenza virus (SIV) or swine-origin influenza virus (S-OIV) refers to any strain of the influenza family of viruses that is endemic in pigs. As of 2009, identified SIV strains include influenza C and the subtypes of influenza A known as H1N1, H1N2, H2N1, H3N1, H3N2, and H2N3.
The swine influenza virus is common throughout pig populations worldwide. Transmission of the virus from pigs to humans is rare and does not always lead to human illness, often resulting only in the production of antibodies in the blood. If transmission causes human illness, it is called a zoonotic swine flu. People with regular exposure to pigs are at increased risk of swine flu infections.
Around the mid-20th century, the identification of influenza subtypes was made possible, allowing accurate diagnosis of transmission to humans. Since then, only 50 such transmissions have been confirmed. These strains of swine flu rarely pass from human to human. Symptoms of zoonotic swine flu in humans are similar to those of influenza and influenza-like illness and include...