Exactly. The descriptions of the deaths during the various and sundry plagues in Europe do show that Yersinia Pestis was the culprit. (Of course, the plague that devasted ancient Athens is still a mystery.) There were also numerous descriptions of dead rats just before the Black Death arrived. (Perhaps the Christian association of cats with witchcraft was a problem; on the other hand, dogs can make good ratters too.)
Rome seemed to do well (in its heyday) because the city fathers concentrated on garbage removal and on having lots of water. General hygene helps controll all the plagues.
There seems, in the historical record of the infamous 14th century outbreaks of plague, to be little doubt that, regards the European phases of the pandemic, the origin was in the Crimea, and spread westward by trading ships. Evidently (I don't have any dog in this fight, btw), Rattus rattus, while enormously adaptable to changes in environment, is also somewhat fragile regarding abrupt climate change. This would account for two things: 1) their supplantation by the Norwegian rat, Rattus norvegicus, in most areas where the two have competed directly, and 2) their exceptionally rapid dying off on occasion in newly habituated areas.
After all, what's a flea to do, eh? The host dies, gee, the flea had better go bite something/someone else, right? However, there's a wildcard here, that, afaik, medical science doesn't as yet have a clue concerning. Specifically, even with dying host animals, the several species of fleas do NOT become either aggressive toward or infectious to mammals until and unless one particular gene, the hemin storage gene, ''switches on'' as it were.
Figure this one out, Doc, and the Nobel Prize is ALL yours!
Best regards and a very Happy New Year to you!
(btw...interesting concise discussion of the biological traits of Rattus rattus at: http://animaldiversity.ummz.umich.edu/site/accounts/information/Rattus_rattus.html )