According to the CDC, it has typically received reports of approximately one human swine influenza virus infection every one to two years in the U.S., but from December 2005 through February 2009, 12 cases of human infection with swine influenza were reported. Presently there have been 40 cases in the U.S. in the past few days, but all are recovering, the CDC stated.

Probably the most well-known outbreak of swine flu was among soldiers in Fort Dix, New Jersey in 1976. CDC documents stated that the virus caused disease, with x-ray evidence of pneumonia, in at least four soldiers and one death. All of these patients had previously been healthy. The virus was transmitted in 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.

The source of the virus, the exact time of its introduction into Fort Dix, and factors limiting its spread and duration were unknown, according to the CDC. The Fort Dix outbreak may have been caused by introduction of an animal virus into a stressed human population in close contact in crowded facilities during the winter, the CDC stated.