The Black Death was a deadly plague that was spreading around Europe during the time of the Hundred Years' War. This disease came from central and eastern Asia. It was brought to Europe from ports that the traders were traveling on. In those ports were nasty little rats carrying the disease. They had fleas on them that feasted on infected blood. Because the environment of Europe was dirty, it was a perfect place for the rats to stay. So the disease didn't stop spreading for awhile.
This disease was so severe and serious that it killed about 25 million people; most people died in Europe because of this, but some people around the world died from this also. The disease wasn't only in Europe, but also in Asia. Because so many people died, there weren't enough workers to stay in the manor system. Those who survived collected enough money, and fled the town. This really changed Europe.