Long ago in Medieval Europe, a terrible disease called the Black Death spread around the country, killing millions.
Before the plague, everyone worked under a system called Feudalism and everything was (mostly) peaceful.
The plague started in Asia, but fleas on rats came from Traders. Also, a man named Genghis Khan used Biological Warfare to speed up the spreading.
He catapulted the bodies of his diseased soldiers into castles. Everyone panicked and shut themselves in their houses, hoping to avoid the disease. Little did they know that the plague spread better in crowded, closed areas.
Everyone began to become ill and most of the lower class died. The social structure faded away and everything became chaos.