Most historians and microbiologists identify the disease that spread in the fourteenth century as the bubonic plague. The disease normally afflicts rats. Fleas living on the infected rats drink their blood and then pass the bacteria that cause the plague on to the next rat they bite. Usually the disease is limited to rats and other rodents, but at certain points in history the fleas have jumped from their rodent hosts to humans and other animals.

The classic symptom of the bubonic plague was a growth the size of a nut or an apple in the armpit, in the groin, or on the neck. This was the boil, or bubo, that gave the disease its name and caused agonizing pain. If the bubo was lanced and the pus thoroughly drained, the victim had a chance of recovery. If the boil was not lanced, however — and in the fourteenth century, it rarely was — the next stage was the appearance of black spots or blotches caused by bleeding under the skin. After that, the victim began to cough violently and spit blood. This stage signaled the end, and death followed in two or three days. The coughing also released plague pathogens into the air, infecting others when they were breathed in and beginning the deadly cycle again on new victims.