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. The 14th century had no suspicion that rats and fleas were the carriers, perhaps because they were so familiar. Fleas, a common household pest, were never mentioned in contemporary plague writings, and rats only incidentally, although folklore did associate them with pestilence. The actual plague bacteria remained undiscovered for another five hundred years.