Medieval Apocalypse: Black Death, What was the plague?

Were rats really responsible for the death of nearly 1/3 of Europes population?

The Black Death or the Plague seems to appear literally out of nowhere. Looking for a scapegoat, many Europeans blamed the Jews for poisoning the wells, or they looked upon it as a judgment from God. Barbara F. Harvey (Introduction: The Crisis of the Early fourteenth Century) "Once it had struck… it set Europe on a new path almost totally unrelated to its late medieval past".

An Italian chronicler in Chronica di Matteo Villani gives this narration of the plague "They began to spit blood and then they died-some immediately, some in two or three days, and some in a longer time. And it happened that whoever cared for the sick caught the disease from them, or infected by the corrupt air, became rapidly ill and died in the same way. Most had swellings in the groin, and many had them in the left and right armpits and in other places, one could almost always find an unusual swelling somewhere on the victim’s body.”

One of the problems facing the researcher is that neither the plague nor the victims can be directly studied. Either way, we know that three kinds of plague existed. The Bubonic affected the glands and was not passed human to human; Pneumonic could be spread from coughing, sneezing, and person to person contact. Finally there was septicemic, where the bacilli enter the bloodstream and multiply and destroy the patient so quickly that buboes can’t form. In this septicemic invasion the patient typically dies within 24-36 hours. This form of the plague can’t be passed from human to human either. The only one capable of doing so was the Pneumonic Plague. The cause of Pneumonic plague, is an untreated case of Bubonic Plague that has progressed to the lungs. When it reaches that point, one person is capable of infecting everyone he comes in contact with. So while rodents and rats are the carriers, they were likely not the biggest culprit in the spread of the plague. It would have been infected patients fleeing the plague that might have struck their family, or neighbors, or the infections caused by caring for sick patients.