A richly told story of the collision between nature's smallest organism and history's mightiest empireThe Emperor Justinian reunified Rome's fractured empire by defeating the Goths and Vandals who had separated Italy, Spain, and North Africa from imperial rule. In his capital at Constantinople he built the world's most beautiful building, married its most powerful empress, and wrote its most enduring legal code, seemingly restoring Rome's fortunes for the next five hundred years. Then, in the summer of 542, he encountered a flea. The ensuing outbreak of bubonic plague killed five thousand people a day in Constantinople and nearly killed Justinian himself.In Justinian's Flea, William Rosen tells the story of history's first pandemic—a plague seven centuries before the Black Death that killed tens of millions, devastated the empires of Persia and Rome, left a path of victims from Ireland to Iraq, and opened the way for the armies of Islam. Weaving together evolutionary microbiology, economics, military strategy, ecology, and ancient and modern medicine, Rosen offers a sweeping narrative of one of the great hinge moments in history, one that will appeal to readers of John Kelly's The Great Mortality, John Barry's The Great Influenza, and Jared Diamond's Collapse.