They were blamed for the plague.
In the late Middle Ages not many Jews left Europe altogether. However, many migrated, mainly from Western Europe to Poland and other parts of eastern Europe, because they were already persecuted by the Christian majority. When the Black Death broke out all over western Europe, the Church, looking for a scapegoat, blamed it on the Jewish population. Jews were rounded up and tortured until they "confessed" to poisoning the water supplies and causing the Plague. In some places hysteria caused people to beat to death any Jewish person they came upon in the streets. It became so bad that many Jews began a mass exodus from the Plague infested areas - mainly to Poland, which was not affected by the plague. In Spain Jews were placed under intense pressure to convert to Christianity, and in 1492 the Jews still in Spain were forced to leave the country and go to North Africa and some other areas.
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They were persecuted in Western Europe and moved eastwards to Poland, which was very tolerant in the Middle Ages and early modern times.
Jews in Europe were fearful of the Crusades, mostly because the Crusaders had been driven to the point of fanaticism and often took that out on Jewish communities in Europe or the Middle East. The number of attacks on Jewish ghettos skyrocketed during the years of the Crusades and Jews, unable to legally defend themselves or procure arms, were often slaughtered.
Poland
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Poland - apex