From: Hadley Leggett, WIRED
Published July 1, 2009 07:43 AM

Swine Flu: Just the Latest Chapter in a 91-Year Pandemic Era

The current strain of H1N1 influenza, or swine flu, has people scared because it’s a novel virus that most of the population has never been exposed to. But as a group, H1N1 viruses aren’t new. They’ve been circulating since 1918, when a new strain appeared simultaneously in pigs and humans and killed 40 to 50 million people in a single year.

Over the past 91 years, the virus has jumped back and forth between humans, pigs and birds — and possibly even been resurrected from a laboratory freezer. Taking a historical view of the swine flu is critical to understanding the current pandemic, and future outbreaks, argue scientists in two perspectives published Monday in the New England Journal of Medicine.

"Ever since 1918, this tenacious virus has drawn on a bag of evolutionary tricks to survive in one form or another, in both humans and pigs, and to spawn a host of novel progeny viruses with novel gene constellations," wrote scientists from the National Institute of Allergy and Infectious Diseases in one perspective.

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They say we’ve been living in a "pandemic era." From 1918 to 1957, H1N1 viruses circulated every year as the seasonal flu. Then, in 1957, a new flu virus, H2N2, appeared on the scene and H1N1 disappeared.

"When a new strain comes out, it overwhelms the population," said Shanta Zimmer, an infectious disease expert at the University of Pittsburgh and a co-author of the other perspective. "Because people had no existing immunity, the new virus replicated and H1N1 got immunologically squeezed out."

Nineteen years later, H1N1 resurfaced on a military base in Fort Dix, New Jersey, where it infected 230 soldiers and killed one. Although the 1976 outbreak was quickly contained and never made it off the base, fear of a worldwide pandemic prompted a massive research campaign and immunization program.

Article coninues:  http://www.wired.com/wiredscience/2009/06/swinefluhistory/

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