During the 1330's, an outbreak of deadly bubonic plague occurred in China. At the time, the plague mainly affected rodents, but fleas transmitted the disease to people. Once humans were infected, they, in turn, contaminated others quite rapidly. The bubonic plague's symptoms were fever and painful swelling of the lymph glands, called buboes, which is how the deadly disease got its name. Also, the sickness caused spots on the skin of those infected: first red, and eventually turning black. .
As it is today, when the bubonic plague occurred, China was one of the busiest of the world's trading nations. Therefore, it was really 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, returning from a trip to the Black Sea, a key link in trade with China, carried on board many merchants already dying of plague. In a matter of days, the disease spread through the city, as well as the surrounding countryside. The disease struck and killed people with terrible speed. The Italian writer Boccaccio said its victims often "ate lunch with their friends and dinner with their ancestors in paradise.".
By August of the following year, the plague had spread as far north as England, where people called it "The Black Death" because of the black spots it produced on the skin. A "terrible killer" was loose across Europe, and medieval medicine had nothing to combat it. .
With the coming of winter, the disease seemed to disappear. This is because fleas, which were the main carriers of the bubonic plague, became dormant during the cold months. Each spring, the plague attacked again, killing new victims. After five years 25 million people were dead, more than one-third of Europe's people. .
When it seemed the worst of the plague was over, smaller outbreaks continued. The oncoming of spring meant the wrath of the plague, not just for years, but for centuries.