Ring around the rosie, a pocket full of posies, husha, husha, they all fall down.
A lot of people hold rats responsible for spreading the black plague of the thirteen hundreds, but really it was fleas. The bacteria that caused the plague has mysteriously disappeared so no one can say precisely, but some scientist think it originated in flea stomachs and was spread to whatever they bit. The black plague originated in china, and eventually spread throughout Eurasia, killing one third of the population. Ill-advised medical practices and a lack of personal hygiene/sanitation did not help, but still, no other event in the history of mankind can lay claim to destruction that extensive. The only thing they really got right was that fire could kind of shield people from the plague. They burned fires on the street corners in the hopes that it would dissipate the 'fog of death' brought down by some astronomical event. The Pope of Avignon (there were two Popes, back in the day) was surrounded by a ring of fire at all times and he got out scott free. The black plague was also kind of the origin of biological warfare. Invading armies would sometimes catapult infected corpses over the walls of their enemies in order to weaken them. I've never heard of a rat doing anything like that.