Bubonic plague, when the germs were carrie in the bloodstreams of fleas and if the rats were biten by the fleas, then the rats were infected and the plague was passed on. It is the best-known manifestation of the bacterial disease plague, caused by the bacterium Yersinia pestis. Bubonic plague is 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 70% of patients in 4-7 days.