This is a myth, actually. Not the Jewish practices mentioned, and not the idea that these might be protective against disease generally, but the idea that 1) they were protective against the bubonic plague specifically, which was in fact a flea-borne disease, and 2) that Jews died of plague at lower rates of plague.
The idea that Jews died at lower rates of the plague in the first wave of second pandemic, which is to say the epidemic in the fourteenth century known as the Great Mortality or the Black Death, comes to us from Christian sources claiming this as a way to justify anti-Jewish violence on the grounds that clearly the Jews were causing this plague. These are obviously sources that have an agenda. By this point, the idea of blaming Jews for the world's problems was thoroughly ingrained, so claims of evidence of Jewish "guilt" such as this need to be taken with a large grain of salt. Likewise, we have Christian sources claiming that Jews died at much higher rates of plague, and therefore they were especially hated by God and should be killed to clense society of them, so that God might stop feeling so wrathful. We also have Christian sources trying to avert further violence claiming that Jews and Christians were dying at comparable rates. The few Jewish sources we have seem indicate that the plague was happy to tear through Jewish communities as well as Christian ones.
We also have a convenient test case, because while we think of this as a European plague, that is an enormously eurocentric view. The plague hit Asia first and then Europe and Africa. And at the time, the Middle East and North Africa was predominantly Muslim, as it still is. And Muslims had similar religious and cultural standards of personal cleanliness to Jews of the time. They also had similar burial practices. In other words, they shared the very practices that I keep seeing brought up as the reasons that Jews were protected from the plague. And the Middle East, North Africa, and Muslim Spain all saw a raging plague epidemic.
All of this makes sense, because hand washing bathing and other personal cleanliness habits are not protective against a vector-borne disease like bubonic plague. And jewish burial customs would actually make jews particularly vulnerable, because of a really nasty quirk of plague biology. Y. pestis, The causitive agent of plague, kills every animal involved in its transmission sequence, rodent, human, and flea. And it kills the fleas by stopping up their gut and causing them to starve to death. This makes the fleas ravenously hungry, and means they will bite frantically anything they can get their mouthparts into. When their current host dies and the body begins to cool, they will do everything they can to jump to the next warm body. And if you come along to strip the body to wash it and prepare it for burial soon after death, as is Jewish custom, congratulations, you've just become the flea's new host, and you are about to get an injection of Y .pestis. This makes bubonic plague one of a rare breed, a disease that is still highly contagious after death.
The spread of plague has much more to do with the built environment than it did with personal habits, and in the first wave of the second pandemic, Christians and Jews were living in relatively comparable spaces. This would soon change. As Christians more and more began to force Jews into ghettos, which were by and large ramshackel, overcrowded, and highly permiable to rats, Jews would begin to die at higher rates than their Christian neighbors in later waves of the second pandemic. The second pandemic extended into the Early Modern period where statistics become better than they are from Medieval Europe, and we can actually see this pattern.
I have in fact seen one Jewish ritual practice brought up as potentially protective against plague that actually holds some water, the cleaning and disposal of chametz in the lead up to Pesach, which would have made Jewish homes less attractive to rats temporarily. However, for this to protect a Jewish community the plague would have to arrive there at exactly the right time. Maybe some Jewish communities were so lucky, but not most.
Ultimately Medieval Christians blamed Jews for the plague and killed them not because they noticed Jews dying in smaller numbers, or larger numbers, not because of any evidence based in reality, but because blaming and killing Jews when things went wrong was already a well traveled road.
Also Medieval Christians bathed. The idea that they didn't is an Enlightenment myth. Rates of bathing actually decreased in the Early Modern period, when there was a fairly brief shift in Europe towards seeing bathing as unhealthy, probably because public bathhouses came to be associated with "immoral behavior," and the spread of STDs, including syphilis. But yeah, the Christian former subjects of the Roman Empire did not all wake up one morning and decide bathing sucks actually. They went right on enjoying public bathhouses throughout the Medieval period.
