The Bubonic Plague of the mid-fourteenth century was the most devastating natural disaster in European History. We now know that this plague was caused by rats that were infected by bacteria-carrying fleas. The rats carried the disease as they were unknowingly transported with cargo ships through trade routes. A third of the entire population of Europe is thought to have been killed by this plague.
What caused the Bubonic Plague?
Explanation
Although poor sanitation did cause the population of rats to grow, and the disease spread by human contact and along trade routes, the source of the Plague has been determined to be fleas that were carried by rats.