From 2005 to January 2009, 12 human cases of swine flu were detected in the United States, without deaths occurring, the CDC said. In September 1988, a healthy 32-year-old pregnant woman in Wisconsin was hospitalized for pneumonia after being infected with swine flu and died a week later. And in 1976, a swine flu outbreak in Fort Dix, New Jersey, caused more than 200 illnesses and one death.

1976: Fear of a great plague By PAUL MICKLE / The Trentonian On the cold afternoon of February 5, 1976, an Army recruit told his drill instructor at Fort Dix that he felt tired and weak but not sick enough to see military medics or skip a big training hike. Within 24 hours, 19-year-old Pvt. David Lewis of Ashley Falls, Mass., was dead, killed by an influenza not seen since the plague of 1918-19, which took 500,000 American lives and 20 million worldwide. Two weeks after the recruit's death, health officials disclosed to America that something called "swine flu" had killed Lewis and hospitalized four of his fellow soldiers at the Army base in Burlington County.

The impact of this pandemic was not limited to 1918–1919. All influenza A pandemics since that time, and indeed almost all cases of influenza A worldwide (excepting human infections from avian viruses such as H5N1 and H7N7), have been caused by descendants of the 1918 virus, including "drifted" H1N1 viruses and reassorted H2N2 and H3N2 viruses. The latter are composed of key genes from the 1918 virus, updated by subsequently incorporated avian influenza genes that code for novel surface proteins, making the 1918 virus indeed the "mother" of all pandemics.

In 1918, the cause of human influenza and its links to avian and swine influenza were unknown. Despite clinical and epidemiologic similarities to influenza pandemics of 1889, 1847, and even earlier, many questioned whether such an explosively fatal disease could be influenza at all. That question did not begin to be resolved until the 1930s, when closely related influenza viruses (now known to be H1N1 viruses) were isolated, first from pigs and shortly thereafter from humans. Seroepidemiologic studies soon linked both of these viruses to the 1918 pandemic (8). Subsequent research indicates that descendants of the 1918 virus still persists enzootically in pigs. They probably also circulated continuously in humans, undergoing gradual antigenic drift and causing annual epidemics, until the 1950s. With the appearance of a new H2N2 pandemic strain in 1957 ("Asian flu"), the direct H1N1 viral descendants of the 1918 pandemic strain disappeared from human circulation entirely, although the related lineage persisted enzootically in pigs. But in 1977, human H1N1 viruses suddenly "reemerged" from a laboratory freezer (9). They continue to circulate endemically and epidemically.

MADISON - Wisconsin heath experts have a history of dealing with swine flu and that's helping them understand the current outbreak. In 1988, a Wisconsin woman died from the virus and gave it to other people. Some of the people who worked on that case still work for the Wisconsin Department of Health Services. The 1988 case involved a woman from Burlington. She caught swine flu at the Walworth County Fair while going through the swine exhibition. The woman was eight months pregnant at the time. She was transferred to Milwaukee County Hospital in Wauwatosa and gave birth to a baby girl. The woman died four days after giving birth.

Of the three influenza strains, only A appears to infect humans and animals (birds, swine, horses and seals). Influenza strains are usually species specific, yet both avian and human influenza strains can infect swine. If cells in the pig are co-infected with human and avian virus, reassortment can lead to major changes in the make-up of the virus. This is called antigenic shift, whereby a sudden dramatic change in the viral genome occurs. Major epidemics and pandemics have occurred when either or both of the two major surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA), from birds recombined with other human segments.