Illustration depicting plague victims, being blessed by a priest, from Omne Bonum, a 14th-century encyclopedia currently maintained at the British Library in London. Image online, courtesy Wikimedia Commons
CAUTION: THIS CHAPTER CONTAINS DESCRIPTIONS OF A DEADLY DISEASE
What is this horrible disease that disfigured, and killed, so many people? It is caused by a bacterium called Yersinia pestis. (Follow the link to the Center for Disease Control to see an actual picture of the rod-like bacterium bacillus.)
Only one bacterium is enough to fatally infect someone!
The bacteria is found in rodents (the black rat during the great pestilence) and is transmitted from rats (and other rodents) to people by Xenopsylla cheopis (the oriental rat flea).
Fleas (which jump from rodents to people) parasitically lodge themselves on the backs of their hosts (the rodents). The fleas' ready food source is the blood of the host. (Note how the flea depicted in the link is engorged with its food.)
If the host is infected, the flea is likely to get infected. When the host dies, the flea needs another source of food.
That's when the trouble for people really begins.