There are actually three forms of Bubonic Plague, which are somewhat dependent on the state of the immune system of the patient. The bubonic form is the least fatal, as there is still an effective lymphatic system which collects the toxins/bacteria in the lymph nodes which swell up and turn black. The second highly fatal form is septicemic which basically poisons the blood in a person with weak immunity. The third highly fatal pneumonic/hemorrhagic form is transmitted from person to person through the air. Here is the Center for Disease Control on the subject.
Bubonic plague is endemic among the wild rodent populations of something like 17 western states. The Black Rat in Europe seems to have been particulaly effective in spreading the disease as it lived upstairs with the people. The Norwegian gray rat which is so common in our cities is a cellar and sewer dweller where it and its fleas comes into less contact with people.
If we were in crisis with a major outbreak, I would lay in a supply of vitamin C and the C complex substance called Pycnogenol, not to mention other immune enhancers.
Pity nobody knows for certain what happened. Does not say good things about control factors on these supposedly controlled items.