THE NOVEL 2009 INFLUENZA A(H1N1) PANDEMIC VI-rus has been an unexpected trigger for pandemic preparedness plans in the United States and elsewhere. It is appropriate to ask how the novel virus might behave epidemiologically in coming months, including the possibility of multiple recurrences or "waves." Spring circulation of the novel virus in the Northern Hemisphere at the end of the 2008-2009 influenza season inevitably has led to comparisons with events in 1918-1919, which in some settings were preceded and followed by outbreaks of respiratory illnesses. Some also believe that the 1918 pandemic began with a premonitory "herald wave," a term related to an old hypothesis, which influenza and dengue fever appeared to have supported, that as new viruses begin to circulate in human populations they inevitably acquire mutations that increase transmissibility and virulence.
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