Was it so back then or might that have been during a warming period? And while it may have been too cold even in the house for fleas to breed on pets and vermin, hygiene was not high on the list of traits of those ancestors. I'm sure humans carried enough fleas and kept themselves warm enough for them to breed. I suspect that once the disease had a foothold, humans were fully capable of doing the rats' work to spread it. Heck, if the court met on an extraoridnarily frequent basis, there's your vector!!! The unusually chummy folk ensured the spread!
But in fact the notion that a pneumonic form of plague -- spread by sneezing and coughing, and not dependent on fleas, has been around for a long time. I guess one of the questions is: Did the Bubonic Plague take two forms, or were there two different diseases?