The official church policy at the time was to protect Jews because Jesus was born into the Jewish race. In practice, Jews were often targets of Christian loath-ing. As the plague swept across Europe in the mid-14th century, annihilating nearly half the population, people had little scientific understanding of the disease and were looking for an explanation.
Jews were often taken as scape-goats and accusations spread that Jews had caused the disease by deliberately poisoning wells. This is likely because they were affected less than other people, since many Jews chose not to use the common wells of towns and cities and because Jews confessed to poisoning wells under torture.
The first massacres directly related to the plague took place in April 1348 in Toulon, Provence where the Jewish quarter was sacked, and forty Jews were murdered in their homes; the next occurred in Barcelona. In 1349, massacres and persecution spread across Europe, including the Erfurt massacre, the Basel massacre, massacres in Aragon, and Flanders. 2,000 Jews were burnt alive on 14 February 1349 in the "Valentine's Day" Strasbourg massacre, where the plague had not yet affected the city. While the ashes smoldered, Christian residents of Strasbourg sifted through and collected the valuable possessions of Jews not burnt by the fires.
Many hundreds of Jewish communities were destroyed in this period. Within the 510 Jewish communities destroyed in this period, some members killed themselves to avoid the persecutions. In the spring of 1349 the Jewish community in Frankfurt am Main was annihilated. This was followed by the destruction of Jewish communities in Mainz and Cologne. The 3,000 strong Jewish population of Mainz initially defended themselves and managed to hold off the Christian attackers. But the Christians managed to overwhelm the Jewish ghetto in the end and killed all of its Jews.
At Speyer, Jewish corpses were disposed in wine casks and cast into the Rhine. By the close of 1349 the worst of the pogroms had ended in Rhineland. But around this time the massacres of Jews started rising near the Hansa townships of the Baltic Coast and in Eastern Europe. By 1351 there had been 350 incidents of anti-Jewish pogroms (deadly riots) and 60 major and 150 minor Jewish communities had been exterminated.
All of this caused the eastward movement of Northern Europe's Jewry to Poland and Lith-uania, where they remained for the next six centuries. King Casimir III of Poland enthusiastically gave refuge and protection to the Jews. The motive for this action remains unclear. The king was well disposed to Jews and had a Jewish mistress. He was also interested in tapping the economic potential of the Jewry.
There are many possible rea-sons why Jews were accused to be the cause for the plague. One reason was because there was a general sense of anti-Semitism in the 14th century. Jews were also isolated in the ghettos, which meant in some places that Jews were less affected. Additionally, there are many Jewish laws that promote cleanliness: a Jew must wash his or her hands before eating bread and after using the bathroom, it was customary for Jews to bathe once a week before the Sabbath, a corpse must be washed before burial, and so on. (Lev. 13:5 also recommends quarantine.)
In many cities the civil authorities either did little to protect the Jewish communities or actually abetted the rioters.
Pope Clement VI (the French-born Benedictine, Pierre Roger) tried to protect the Jewish communities issuing two papal bulls in 1348 (the first on 6 July and another 26 September) saying that those who blamed the plague on the Jews had been "seduced by that liar, the Devil". He went on to emphasize that “It cannot be true that the Jews, by such a heinous crime, are the cause or occasion of the plague, because through many parts of the world the same plague, by the hidden judgment of God, has afflicted and afflicts the Jews themselves and many other races who have never lived alongside them.” He urged clergy to take action to protect Jews and offered them papal protection in the city of Avignon. In this, Clement was aided by the researches of his personal physician Guy de Chauliac who argued from his own treatment of the infected that the Jews were not to blame.
Clement's efforts were in part undone by the newly elected Charles IV, Holy Roman Emperor making property of Jews killed in riots forfeit, giving local authorities a financial incentive to turn a blind eye.
As the plague waned in 1350, so did the violence against Jewish communities. In 1351, the plague (killing 200 million) and the immediate persecution was over, though the background level of persecution and discrimination remained. Ziegler (1998) comments that "there was nothing unique about the massacres." 20 years after the Black Death the Brussels massacre (1370) wiped out the Belgian Jewish community.
(This article in Wikipedia is fully footnoted.)