Yersinia Pestis

The true killer was a bacterium called Yersinia pestis. It lived in fleas, the fleas lived on rats, and the rats traveled wherever people and cargo went.

The outbreak rode west along trade routes. Genoese ships left Caffa, a port in Crimea, with infected sailors and rats below deck. When they docked at Messina in Sicily in 1347, the plague stepped ashore with them — and from that single harbor it spread across the continent.