The Great War may have unwittingly enabled that era's other mass killer: influenza. The great flu pandemic of 1918 - which modern scientists study as a model of what could happen in the event of a “bird flu” outbreak — killed up to 675,000 Americans, 0.65 percent of the nation's population. Most died in a terrifying span of 16 weeks. Worldwide, the toll was far worse: Death estimates range from 21 million to 100 million.

John M. Barry, author of The Great Influenza (Viking, $29.95), speculates the deadly virus may have first appeared in Haskell County, Kan., from which newly mustered soldiers spread it to Camp Funston, Kan., in February 1918. The US Army's gathering of men from across the country into close quarters galvanized the flu: In the spring of 1918, 24 of the 36 largest army camps suffered outbreaks. Barry adds that 30 of the 50 largest US cities - notably, those close to Army bases — also saw outbreaks.