The flu arrived as a great war raged in Europe, a conflict that would leave about 20 million people dead over four years. In 1918, the flu would kill more than twice that number — and perhaps five times as many — in just 15 months. Though mostly forgotten, it has been called “the greatest medical holocaust in history.” Experts believe between 50 and 100 million people were killed. More than two-thirds of them died in a single 10-week period in the autumn of 1918. Never have so many died so swiftly from a single disease. In the United States...