Original: Swine influenza, also called pig influenza, swine flu, hog flu and pig flu, is an infection by any one of several types of swine influenza virus. Swine influenza virus (SIV) or S-OIV (swine-origin influenza virus) is any strain of the influenza family of viruses that is endemic in pigs.
Simplified: Swine influenza virus (often shortened to SIV or swine flu) is one of a number of viruses that are endemic to pigs.