LONDON, U.K.—The Black Death killed about 30 per cent of the European population in a few years in the middle of the 14th century. A century and a half later, the native people of the Americas were hit by half a dozen plagues as bad as the Black Death, one after another, and 95 per cent of them died. The plagues of the “Great Dying” had much less terrifying names like measles, influenza, diphtheria, and smallpox, but they were just as efficient at killing.