Swine influenza is an infection caused by any one of several types of swine influenza viruses. Swine influenza virus SIV or swine-origin influenza virus S-OIV is any strain of the influenza family of viruses that is endemic in pigs. The Swine flu was initially seen in humans in Mexico in , where the strain of the particular virus was a mixture from 3 types of strains. Swine influenza virus is common throughout pig populations worldwide. Transmission of the virus from pigs to humans is not common and does not always lead to human flu , often resulting only in the production of antibodies in the blood. If transmission does cause human flu, it is called zoonotic swine flu.