Swine influenza (swine flu) is a respiratory disease caused by viruses carried by pigs – in wild populations and in livestock all around the world. U.S. populations are no exception with at least 30% of the pigs tested domestically having shown antibody evidence of a previous infection with one of the viruses. The disease is not usually fatal or even serious. Symptoms may be lethargy, decreased appetite, and a runny nose with a cough. Many swine show no detectable symptoms at all. Swine flu viruses can be transmitted to humans who have direct contact with infected pigs, but this is very rare.