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 — 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.