1) In 1347, a plague known as the Black Death spread from Asia to Europe through trade routes, killing over 20 million people within five years as it spread across the continent.
2) Due to the lack of medical knowledge at the time, many Christians believed the plague was a divine punishment from God for sins such as corruption in the church or failure to fully participate in the Crusades.
3) This belief led Christians to persecute groups they deemed responsible for their sins, most notably Jews, resulting in massacres of thousands across Europe as a way to regain God's favor and stop the plague.