During a devastating nineteenth-century cholera outbreak, English physician John Snow proved that the deadly disease could hide in a drop of water. In the twentieth century, burgeoning cities would subdue cholera and typhoid by building massive filtration plants and bubbling poisonous gas through their drinking water. But in the new millennium, the demon of waterborne disease is threatening to reemerge, and the results could be catastrophic. In this fascinating, sobering account, Dr. Robert Morris depicts the epidemics that have shaken nations, celebrates the scientists who reached into the invisible and ultimately saved millions of lives, and sounds a timely warning we dare not ignore about the natural and man-made hazards present in the water we drink.