What the world at this time didn't know was that this was bubonic plague, a contagious, fatal epidemic disease caused by the bacterium Pasturella pestis and carried by infected fleas, which traveled by virtue of Rattus rattus, the small medieval black rat that lived on ships, as well as the heavier brown or sewer rat. The presence of both at once caused an extremely high mortality rate and a quick spread of contagion. The malignancy of the pestilence appeared particularly terrible because its victims had no idea of what was killing them, knew no prevention, and had no remedy. This infection is transmitted from person to person by normal contact or by the bite of fleas from an infected host. The blood stream of the bitten or infected person then carries the bacteria throughout the body.
Bacteria invade lymph nodes, which swell, creating the buboes.
The infection is also spread by respiratory contact after the infection has spread to the lungs. This is Pneumonic Plague, the most contagious form of the disease. Bacteria invade the victim's lungs, which fill with frothy bloody liquid. In either case, death usually occurs in less than three days. Ignorance of this medical knowledge only augmented the horror of the time.