In 1894, during an outbreak of disease in Hong Kong and India known as the Third Pandemic, bacteriologists Alexander Yersin and Shibasaburo Kitasato, working independently of each other, identified the bacteria that caused plague. This bacterium came to be called Yersinia pestis, when Yersin showed it to be the causative agent of the plague in India. Working backwards, Yersin determined that plague was the cause of the Black Death as well, due to the medieval records of large tumors.
Yersinia pestis is usually transmitted from rodent to flea to rodent. Humans are normally only targeted by fleas when there are no rodents left. When a flea bites a healthy rodent, the blood from the rodent goes directly to the flea's stomach, easing hunger. But when a flea bites a rodent infected with Y. pestis, the bacterium-riddled blood gets stuck in the flea's foregut. The bacteria will grow, engorging the flea. The flea constantly feels hungry because nothing is getting to its stomach. In response to hunger pangs, the flea feeds greedily on more rodents. It spreads the disease by regurgitating the infected blood into healthy rodents. When the rats start to die off, fleas swarm the remaining rodents. Finally, when all the rats have died, the fleas turn to people.