The history of "the greatest massacre of the twentieth century," an illness that infected more than 500 million people.Between 1918 and 1920, the "Spanish flu" killed more than 50 million people, far more than in the world war then raging. Unlike the familiar flu, which targets infants and the elderly, it killed healthy adults. It was mankind's worst epidemic, writes Paris-based science journalist and novelist Spinney (The Quick, 2007, etc.) in this fine account of influenza's history, its worst attack (so far), and its ominous future. Despite the name, Americans were probably the first to experience the fever, cough, headache, and general miseries of the infection. During spring and summer, it behaved like the usual flu, but in fall 1918, it turned deadly and spread across the world, killing 2.5 to 10 percent of victims, a fatality rate 20 times higher than normal. Scientists have offered countless theories about the illness, but Spinney looks favorably at a recent theory that the 1918 virus provoked a "cytokine storm," a deadly overreaction of the immune system. This may explain why infants and the elderly, with their weaker immune systems, had an easier time. In the middle sections of the book, the author describes how a dozen nations dealt with the epidemic. Heroism was not in short supply, but superstition, racism, ignorance (including among doctors), and politics usually prevailed. In the concluding section, Spinney recounts impressive scientific progress over the past century but no breakthroughs. Revealing the entire viral genome opens many possibilities, but so far none have emerged. Researchers are working to improve today's only modestly protective vaccine; Spinney expresses hope. Readers who worry about Ebola, Zika, or SARS should understand that epidemiologists agree that a recurrence of the 1918 virus would be worse. Short on optimism but a compelling, expert account of a half-forgotten historical catastrophe.