Somewhere in Asia, probably due to an earthquake, fleas, carrying a deadly disease were disturbed. The disease then began to spread to other animals (like rats) - and humans
When Caffa, a fortress in Russia was attacked, the invaders biffed bodies infected with the disease (Now known as the Black Death) into the city to infect the people. It worked. It did not take long before the people of Caffa had been infected.
Sailers fled from Caffa, as well as many stowaway rats and fleas.
The sailors went from port to port, but nobody would let them dock, for fear of infection. Meanwhile, rats and fleas ran rampant on their boat.
Eventually, the sailors docked in Sicily for a short period of time. Then they were rebuffed, but not before infected rats and fleas had flown and crawled onto the dock, and into Italy.
As the infected rats and fleas spread into Europe, they passed on the disease to humans. Fleas and rats multiplied and thrived in the overcrowded and unclean homes. As the disease spread, killing millions, panic and blame spread throughout Europe.