How was the Black Death transmitted?

The three forms of the Black Death were transmitted two ways. The septicemic and bubonic plague were transmitted with direct contact with a flea, while the pneumonic plague was transmitted through airborne droplets of saliva coughed up by bubonic- or septicemic-infected humans.

The bubonic and septicemic plagues were transmitted by the the bite of an infected flea. Fleas, humans, and rats served as hosts for the disease. The bacteria (Yersinia pestis) multiplied inside the flea, blocking the insect’s stomach and causing it to be very hungry. It would then start voraciously biting a host. Since the feeding tube to its stomach was blocked, the flea was unable to satisfy its hunger. As a result, it continued to feed in a frenzy. During the feeding process, infected blood carrying the plague bacteria flowed into the human’s wound. The flea soon starved to death, but the plague bacteria now had a new host.

The pneumonic plague was transmitted through droplets sprayed from the lungs and mouth of an infected person. In these droplets were the bacteria that caused the plague. This bacteria entered the lungs through the windpipe and began to attack the lungs and throat.