As a deadly new strain of avian influenza spreads from country to country, researchers hunt the virus down in the most remote regions of the world while scientists look to the past in order to prepare for the future. The clues needed to prevent a global pandemic, they believe, are locked in the long-dormant genes of one of the greatest plagues in history—the 1918 Spanish flu. History’s killer flu, they discover, also started as a bird flu.