Swine influenza (also called swine flu, hog flu, and pig flu) is an infection of a host animal by any one of several specific types of microscopic organisms called "swine influenza virus".

A swine influenza virus (SIV) is any strain of the influenza family of viruses that is usually hosted by pigs. As of 2009, the known SIV strains are the influenza C virus and the subtypes of the influenza A virus known as H1N1, H1N2, H3N1, H3N2, and H2N3.

Swine influenza is common in pigs in the mid-western United States, Mexico, Canada, South America, Europe (including the United Kingdom, Sweden, and Italy), Kenya, China, Taiwan, Japan and other parts of eastern Asia.

In humans, the symptoms of swine flu are similar to those of influenza and of influenza-like illness in general, namely chills, fever, sore throat, muscle pains, severe headache, coughing, weakness and general discomfort.

The 2009 swine flu outbreak in humans is due to a new strain of influenza A virus subtype H1N1 that contains genes closely related to swine influenza. The origin of this new strain is unknown.

However, the World Organisation for Animal Health reports that this strain has not been isolated in pigs. This strain can be transmitted from human to human, and causes the normal symptoms of influenza.