From what I remember reading years ago, the real problem with plague, aside from fleas, was the black rat (Rattus rattus) which nowadays has been largely displaced by the brown rat (Rattus norvegicus).
It’s been so long, but I believe it had something to do with the black rat’s nesting behavior in human habitations. Simply put, the black rat is more an indoors sort of critter, while the brown rat is more outdoorsy. Needless to say, black rats, indoors, with plague-carrying fleas, is a recipe for disaster.
The California tree rat, which loves to inhabit the mats of dead fronds on the trunks of Washington palm trees, so commonplace in California is, from what I understand, the same species (rattus rattus) as the plague rat. The brown rat (rattus norvegucus) is also known as the New York City sewer rat.
Another part of the problem with the black plague outbreak in the 14th century was the cultural bias against cats. popular folk beliefs had cats depicted as in league with the devil. In many towns cats were hunted down and eradicated. Unfortunately that allowed the vermin population to explode which made the plague situation exponentionally worse. Unintended consequences and all that.
CC
Not to mention people also decided that cats were evil and were killing them for fun...the kitties could have saved some of those people from a horrific death...