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 Simonson, an epidemiologist at the National Institutes of Health, 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, Simonson suggests there may be time to develop an effective vaccine before a second, more virulent strain, begins to circulate.