Black Death, pandemic that ravaged Europe between 1347 and 1351, taking a proportionately greater toll of life than any other known epidemic or war up to that time. The Black Death is widely believed to have been the result of plague, caused by infection with the bacterium Yersinia pestis. Modern genetic analyses indicate that the strain of Y. pestis introduced during the Black Death is ancestral to all extant circulating Y. pestis strains known to cause disease in humans. Hence, the origin of modern plague epidemics lies in the medieval period. Other scientific evidence has indicated that th...