A few years ago, in a discussion of Antisemitism in European history, my old history teacher mentioned that during the first great Bubonic Plague outbreak in Europe, many Jewish communities suffered far fewer deaths than in the general population. He stated that this was due to the far more sanitary conditions in most Jewish communities, which he said was due to traditional Jewish customs of cleanliness. He also said that Jewish communities suffered backlash and even pogroms in some areas as a result of the plague, because it was observed that there were fewer Jewish victims, and as a result some people came to believe that Jews had called down the plague on the Christian population.
There was already ample antisemitism in Europe before the plague. There were a variety of reasons for this, but one I find particularly common is that money-starved leaders saw Jews as easy targets. In 1289, the leaders of Gascony expelled the Jews from the region and confiscated their property. The following year, the English Crown did the same. In France, Jews were thrown out in 1306, 1322 and 1394.
There were, of course, nasty religious and cultural reasons for antisemitism. The Crusaders' holocaust of 1096 is well-known, but then there's the blood libel accusation that lingered into the 19th century. That accusation states that Jews kill Christian children to use their blood in arcane rituals.
At the Fourth Lateran Council (1215), church authorities ruled that Jews "at all times shall be marked off ... through the character of their dress." There were badges and stripes of all kinds and colors to do this. Jews were not the only people singled out by a special dress code, but it's an insidious antisemitic move ─ it allows aggressors to pick their targets more easily.
As John Kelly writes in The Great Mortality, there were pogroms or outbreaks of antisemitic violence in 1146, 1189, 1217, 1288, 1298 and 1321. "The last of these, the pogrom of 1321, was notable for being a kind of dress rehearsal for the anti-Semitic violence of the Black Death," Kelly writes.
In both cases, there was Holy Week violence (around the time of Easter), rumors of poisoned wells, and whispers of secret cabals. On July 6, in a papal letter intended to quell the violence, Pope Clement VI wrote that the Jews could not be the cause of the plague because they were affected by it themselves.
It didn't help much.
"Increasingly, in villages and forest clearings," Kelly writes, "men and women told one another: Perhaps if we kill all the Jews, the plague won't come to our village. Even if the plague did come, with the Jews dead, at least debts to Jewish moneylenders would be canceled. Later, after the violence subsided, one chronicler would write that the 'poison which killed the Jews was their wealth.'"
What I've read -and I think it was here on Reddit- is that Jewish communities had a lot of cats (seems like cats were common in Jewish households back then).
Cats would kill the rats, and that would help keeping the plague away.