Swine influenza (swine flu) is a respiratory disease of pigs. Swine flu is caused by Type A influenza virus, which is the only type of influenza virus to have caused human pandemics. Swine flu outbreaks in pigs occur regularly, causing high levels of illness and low death rates. Swine influenza viruses may circulate among swine throughout the year, but most outbreaks occur during the late fall and winter months similar to outbreaks in humans. The swine flu virus is an H1N1 virus that occasionally infects people without causing large outbreaks. Only twelve cases of H1N1 (swine flu) in humans were reported in the United States over the last four years (January 2005 through January 2009). None of them caused deaths. An outbreak of H1N1 occurred among soldiers in Fort Dix, New Jersey, in 1976. At least four soldiers became ill with swine flu and one died; all of these patients had previously been healthy. The virus was transmitted to close contacts in a basic training environment, with limited transmission outside the basic training group. The virus is thought to have circulated for a month and disappeared.