Colorized Transmission Electron Micrograph (TEM) of negatively stained SW31 (swine strain) influenza virus particles. Swine influenza, also called pig influenza, swine flu, hog flu and pig flu, 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. 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. People with regular exposure to pigs are at increased risk of swine flu infection. Symptoms of zoonotic swine flu in humans are similar to those of influenza; chills, fever, sore throat, muscle pains, severe headache, coughing, weakness and general discomfort.