Bubonic plague is the best known manifestation of the bacterial disease plague, caused by the Gram-negative bacterium Yersinia pestis (formerly known as Pasteurella pestis). It belongs to the family Enterobacteriaceae. The term "bubonic plague" was often used synonymously for plague, but it does in fact refer specifically to an infection that enters through the skin and travels through the lymphatics, as is often seen in flea-borne infections. Bubonic plague kills about half of infected patients in 3–7 days without treatment, and may be the Black Death that swept through Europe in the 1340s, killing tens of millions.