During the Middle Ages Christians believed Jews were associated with the devil and were out to eliminate Christianity. Christians believed Jews were guilty of all the ills of the world, regardless of how preposterous and irrational the claims.
When the plague hit Europe in 1348 it killed 20-25 million people, about a third of Europe’s population. For a Christian population who had already accused Jews of poisoning wells, it was natural to assume they also brought the plague. The fact that Jews died as well as Christians did not deter ordinary Christians from thinking this, despite the fact that Pope Clement VI pointed out this obvious fact.