The Black Death Massacres And Persecutions Against Jews
The 14th century was a dangerous time to be Jewish in Europe. Well, it almost always is, but that was the time of the Black Death, so in addition to avoiding the Plague, Jews in Europe had to contend with crazed and paranoid neighbors who blamed them for the state of affairs. A lucky few managed to escape to the safe havens of Poland and Lithuania, but many more were burned at the stake or otherwise killed en masse, thanks to the senseless prejudice that spread alongside the disease.
Poisoning The Well
Struggling to understand, in the time before germ theory, why their loved ones were dropping like flies, many Christians in Europe decided—like most things, as far as they were concerned—it was all the Jews' fault. Jews were already seen as enemies of Christianity, with attacks on them dating back to the Crusades of 1096, so unfortunately, violent antisemitism was old hat to many Christians. It didn't help that, in the 12th and 13th centuries, the Talmud was destroyed for containing what was believed to be secret messages, so this particular faction of Christians was already primed to believe in a Jewish conspiracy.
Specifically, they came to believe in a network of Jewish communities plotting to literally poison their wells. Their only evidence of the theory was that Jews rarely used the common wells that were said to have been poisoned and didn't seem to catch the disease as often as others, though this is probably because Jewish communities were largely segregated and their religious laws were strict about cleanliness. Jews who were detained and tortured also confessed to poisoning the wells with frogs, lizards, spiders, and "Christians' hearts," but those confessions can be confidently dismissed. Pope Clement VI saw the writing on the wall and issued a statement that the Plague affected people of all races and religions, but not even he could stop the antisemitic train that was about to roll through Europe.