Just a note here to mention that the Black Death that killed a third of Europeans in 1347-1349 really was bubonic plague. There have always been skeptics who say the disease just didn't spread like bubonic plague does, that many of the symptoms described by contemporaries are not plague symptoms, and so on. But now soil and bones from plague burial pits have been tested, and they contain massive amounts of DNA from the plague bacillus, Yersinia pestis. Enough DNA has been found in some places to show that at least two variants of plague are present, and that they are the same variants found in Chinese samples. This is not to say that there might not have also been other diseases running rampant populations weakened by the plague, or spread by the hordes of plague refugees, but we know for certain now that the heart of the problem was bubonic plague.
Sometimes history does make progress, and old questions are given definitive answers.