Swine flu virus. Transmission electron micrograph (TEM) of SW31 (swine strain) influenza (flu) virus particles (round, orange) near an MDCK cultured cell (green). Swine flu is an infection caused by any one of several types of flu viruses that are endemic to pigs. Swine influenza virus (SIV) is common throughout pig populations worldwide. However, 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. People with regular exposure to pigs are at increased risk of swine flu infection.