Swine flu: A respiratory illness of pigs caused by infection with swine influenza A virus (SIV). While swine flu viruses normally do not infect humans, occasional infections of humans do occur. Human cases of swine influenza A virus infection occur in individuals who have had a history of recent direct contact with pigs or close (within 6 feet) contact with pigs. Rare instances of human-to-human transmission have been documented. Swine flu infections have also occurred in individuals with no history of exposure to pigs. Symptoms typically range from a mild respiratory illness to flu-like symptoms with fever. Treatment involves the use of antiviral medications begun as soon as possible after the onset of symptoms. Vaccines are available to be given to pigs to prevent swine flu in these animals.
The swine flu pandemic of 2009 is caused by a novel influenza A (H1N1) virus that was originally referred to as "swine flu" because many of the genes in this new virus were very similar to influenza viruses that normally occur in pigs in North America. However, this new virus is actually quite different from the typical swine flu viruses found in pigs; the virus responsible for the 2009 pandemic is termed H1N1 flu virus or novel H1N1 flu virus. This virus first caused illness in Mexico and the United States in March and April, 2009. H1N1 flu is spread from person to person, unlike typical swine flu as described above. In 2009 vaccines are being developed for the prevention of swine flu in humans.