Background

The swine flu is likely a descendant of the infamous "Spanish flu"[citation needed]that caused a devastating pandemic in humans in 1918–1919. In less than a year, that pandemic killed more an estimated 50 million people worldwide. Descendants of this virus have persisted in pigs; they probably circulated in humans until the appearance of the Asian flu in 1957, and reemerged in 1977.Direct transmission from pigs to humans is rare, with 12 cases in the U.S. since 2005.

The flu virus is perhaps the trickiest known to medical science; it constantly changes form to elude the protective antibodies that the body has developed in response to previous exposures to influenza or to influenza vaccines. Every two or three years the virus undergoes minor changes. Then, at intervals of roughly a decade, after the bulk of the world's population has developed some level of resistance to these minor changes, it undergoes a major shift that enables it to tear off on yet another pandemic sweep around the world, infecting hundreds of millions of people who suddenly find their antibody defenses outflanked.Even during the Spanish flu pandemic, the initial wave of the disease was relatively mild and the second wave was highly lethal.

In 1957, an Asian flu pandemic infected some 45 million Americans and killed 70,000. Eleven years later, lasting from 1968 to 1969, the Hong Kong flu pandemic afflicted 50 million Americans and caused 33,000 deaths, costing approximately $3.9 billion. In 1976, about 500 soldiers became infected with swine flu over a period of a few weeks. However, by the end of the month investigators found that the virus had "mysteriously disappeared" and there were no more signs of swine flu anywhere on the post. There were isolated cases around the U.S. but those cases were supposedly to individuals who caught the virus from pigs.

Medical researchers worldwide, recognizing that the swine flu virus might again mutate into something as deadly as the Spanish flu, were carefully watching the latest 2009 outbreak of swine flu and making contingency plans for a possible global pandemic.