Then famine entered overpopulated Europe. Between 1315 and 1317 torrential rains destroyed crops. As hunger and death stalked the land, people were driven to cannibalism. They even opened graves to claim the newly dead for food. It's an old story.
Good times increase population until it has little capacity to withstand bad times.
But this time famine was only the beginning. In AD 1347 Tatars attacked a group of European merchants in a town in the Crimea. The attacking Tatars were suffering from an epidemic. Too sick to press their siege, they used their trebuchets to hurl their own dead over the city walls. That primitive germ warfare was devastatingly successful.