The Black Death
During the Hundred Years' War, a lot of people in Europe were very poor and hungry because the soldiers fighting the war had wrecked their farms. Then people began to catch a terrible sickness that was spreading along Mongol trade routes from China through Central Asia to Europe beginning in 1328 AD.
When you caught the plague you got big black spots called buboes on your armpits and behind your knees and on your neck. A lot of people called it the Black Death, because after that the tips of your fingers and toes and your nose often turned black.
Most of the people who got the plague died, especially poor people who didn't have good food to eat. About one out of every three people in Europe, and many people in Asia too, died during this plague.
Partly because of this plague, the Mongol Empire collapsed in Central Asia. It may also be true that because so many people had died in Europe, the survivors were richer and better fed, and that this led to the Renaissance.