In the early 1330s an outbreak of deadly bubonic plague occurred in China. Plague mainly affects rodents, but fleas can transmit the disease to people. Once people are infected, they infect others very rapidly. Plague causes fever and a painful swelling of the lymph glands called buboes, which is how it gets its name. The disease also causes spots on the skin that are red at first and then turn black.

Since China was one of the busiest of the world's trading nations, it was only a matter of time before the outbreak of plague in China spread to western Asia and Europe. In October of 1347, several Italian merchant ships returned from a trip to the Black Sea, one of the key links in trade with China. When the ships docked in Sicily, many of those on board were already dying of plague. Within days the disease spread to the city and the surrounding countryside.