The outbreak began so slowly that no one in Dallas perceived it at first. In June 2012, a trickle of people began showing up in emergency rooms broiling with fever, complaining that their necks were stiff and that bright lights hurt their eyes. The numbers were initially small; but by the middle of July, there were more than 50 victims each week, slumping in doctors’ offices or carried into hospitals comatose or paralyzed from inflammation in their brains. In early August, after nine people died, Dallas County declared a state of emergency: It was caught in an epidemic of what turned out to be West Nile virus, the worst ever experienced by a city in the United States. By the end of the year, 1,162 people had tested positive for the mosquito-borne virus; 216 had become sick enough to be hospitalized; and 19 were dead.