Causes of plague

Causes of plague

We know today that plague is caused by the bacterium Yersinia pestis, which normally infects rats and other rodents. Plague bacteria may live quietly for centuries among rodent populations, as these animals can often harbor the infection without dying. Fleas carry it form one animal to another, transmitting bacteria in their bite. When an infected rodent dies, its fleas seek out another host. If a flea can fine no other rodent to feed on, it turns to the nearest food source--which might be a human being. Rats and fleas were common in human dwellings in the 1300's, so when plague bacteria entered European rat populations, no one suspected these animals as the carriers of a sudden new disease. To the people of the time, it seemd that the deadly infection came from the air itself, or the hand of God.

Based on "Apocalypse Then: A History of Plague." The 1992 World Book Health & Medical Annual, pp. 166-181.