If the plague had just stayed in one city, the containment might have spared Europe. Unfortunately, the plague spread when people fled to other cities.
It is believed the plague originated in Asia, and moved west with Mongol armies and traders.
"According to a traditional story, the plague came to Europe from the town of Caffa, a Crimean port on the Black Sea where Italian merchants from Genoa maintained a thriving trade center. The Crimea was inhabited by Tartars, a people of the steppe, a dry, treeless region of central Asia. When the plague struck the area in 1346, tens of thousands of Tartars died. Perhaps superstition caused the Muslim Tartars to blame their misfortune on the Christian Genoese. Or perhaps a Christian and Muslim had become involved in a street brawl in Caffa, and the Tartars wanted revenge. In any case, the Tartars sent an army to attack Caffa, where the Genoese had fortified themselves. As the Tartars laid siege to Caffa, plague struck their army and many died. The Tartars decided to share their suffering with the Genoese. They used huge catapults to lob the infected corpses of plague victims over the walls of Caffa. As the Tartars had intended, the rotting corpses littered the streets, and the plague quickly spread throughout the besieged city. The Genoese decided they must flee; they boarded their galleys and set sail for Italy, carrying rats, fleas, and the Black Death with them."
. The same with the rats.
