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If there's a blessing in the current swine flu epidemic, it's how benign the illness seems to be outside the central disease cluster in Mexico. But history offers a dark warning to anyone ready to write off the 2009 H1N1 virus.

In each of the four major pandemics since 1889, a spring wave of relatively mild illness was followed by a second wave, a few months later, of a much more virulent disease. This was true in 1889, 1957, 1968 and in the catastrophic flu outbreak of 1918, which sickened an estimated third of the world's population and killed, conservatively, 50 million people.

Lone Simonsen, an epidemiologist at George Washington University, who has studied the course of prior pandemics in both the United States and her native Denmark, says, "The good news from past pandemics, in several experiences, is that the majority of deaths have happened not in the first wave, but later." Based on this, Simonsen suggests there may be time to develop an effective vaccine before a second, more virulent strain, begins to circulate.

As swine flu -- also known as the 2009 version of the H1N1 flu strain -- spreads, Simonsen and other health experts are diving into the history books for clues about how the outbreak might unfold -- and, more importantly, how it might be contained. In fact, the official Pandemic Influenza Operation Plan, or O-Plan, of the U.S. Centers for Disease Control and Prevention, is based in large part on a history lesson -- research organized by pediatrician and medical historian Dr. Howard Markel of the University of Michigan.

Markel was tapped by the CDC to study what worked and what didn't during the 1918 flu disaster. Markel and colleagues examined 43 cities and found that so-called nonpharmaceutical interventions -- steps such as isolating patients and school closings -- were remarkably successful in tamping down the outbreak. "They don't make the population immune, but they buy you time, either by preventing influenza from getting into the community or slowing down the spread," Markel said. Explainer: Flu facts Read full article »

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