Well the thing is Bubonic wasn't the only type of plague recorded in ancient history, or known by modern scientists. There are several other kinds of Pestis bacterium that are spread by other means, that can cause other kinds of plagues. Yersinia pestis can actually cause 3 types of plagues.
Some of the stuff that was called Bubonic plague was actually had symptoms from other plague types that Bubonic plague itself doesn't have.
Some of those other symptoms can also be caused by other things besides different kinds of "pestis" bacteriums.
Thus why historians debate on if there was more than one type of plague going on during the "Black Death" and some of the other great plagues.
Also certain kinds of pestis varieties can also be transmitted through physical contact with soil that harbored the bactyeria, fecal matter, saliva, or physical contact with those that were already infected. We are talking about a time when people dumped their feces right into the street.
So their were other means of transmission, not to mention different varities of pestis plague.
To quote Wikipedia, these are statistics for deaths during the "Black Death" for the various kinds of plagues;
The bubonic plague was the most commonly seen form during the Black Death, with a mortality rate of thirty to seventy-five percent and symptoms including fever of 38 - 41 °C (101-105 °F), headaches, aching joints, nausea and vomiting, and a general feeling of malaise. The pneumonic plague was the second most commonly seen form during the Black Death, with a mortality rate of ninety to ninety-five percent. Symptoms included slimy sputum tinted with blood. As the disease progressed, sputum became free flowing and bright red. Septicaemic plague was the most rare of the three forms, with mortality close to one hundred percent. Symptoms were high fevers and skin turning deep shades of purple due to DIC (Disseminated intravascular coagulation).
Plus some of the symptoms are shared with other kinds of diseases, so there is the chance that some died of something else, but since it shared symptoms with the various pestis plagues, it was documented as being part of the Black Death.
The question is not of if rats with fleas spread the disease, since its largely believed rats were one of the culprits. The real question is how much did the rats or fleas spread the disease, and how much was transmission originated from other causes.
