Few events in European history seem as cataclysmic as the Black Death. The plague — which most historians believe was the bubonic plague, though others have suggested an ebola-like virus — is thought to have originated around the Black Sea region and spread rapidly through much of Europe and parts of Asia among rats nestling in the bowels of grain ships. The pandemic's most devastating moment was a four-year spell between 1347 and 1351 that, according to some estimates, wiped out as much as two-thirds of Europe's population. The plague would recur in subsequent decades and is believed to have killed roughly 100 million people worldwide in the space of some 200 years. Its legacy, particularly in Europe, was traumatic and deep, reshaping the continent's social and economic landscape while burrowing its way into ecclesiastical art — including grisly pictures of devotees covered with pus-filled boils — and medieval literature. The children's nursery rhyme that begins "Ring-a-ring o' roses/ A pocketful of posies" and ends with the line "We all fall down" is an eerie hand-me-down from an age when a grim-reaping plague stalked the land.