Swine influenza (also swine flu) refers to influenza caused by any strain of the influenza virus that is endemic in pigs (swine). Strains endemic in swine are called swine influenza virus (SIV). Of the three genera of Orthomyxoviridae that are endemic in humans, two are endemic also in swine: Influenzavirus A (common) or Influenzavirus C (rare). Influenzavirus B has not been reported in swine. Within Influenzavirus A and Influenzavirus C, the strains endemic to swine and humans are largely distinct.
People who work with poultry and swine, especially people with intense exposures, are at risk of infection with influenza from these animals if the animals carry a strain that is also able to infect humans. SIV can mutate into a form that allows it to pass from human to human. The strain responsible for the 2009 swine flu outbreak is believed to have undergone such a mutation.
In humans, the symptoms of swine flu are similar to those of influenza and of influenza-like illness in general. In most cases, the strain responsible for the 2009 swine flu outbreak causes only mild symptoms.