Slightly off topic but concerning the Black Death, I have been reading a book entitled "Biology of Plagues. Evidence from Historical Populations" by Susan Scott and Christopher Duncan. It make a very convincing case that the modern-day Bubonic Plague and the Black Death are two entirely different diseases, with similar symptoms (most famously the swellings of the lymph nodes called "buboes") but very different causes and epidemiology.
The Bubonic Plague is a bacterial disease caused by Yersinia pestis, carried by fleas. It is rather difficult to catch, was mainly localized to urban areas, and has a relatively low mortality rate. On the other hand, the detailed epidemiology of the Black Death strongly supports a viral vector, with a long latency period (over a month) before overt symptoms very rapidly appear. The epidemiology in no way supports a bacterial vector -- in fact it would be completely unheard of among bacterial diseases. In addition, genetic evidence among the survivor populations (the CCR5-D32 mutation, relating to the viral immune response) suggests that the mutation gave a survival advantage during a series of heavy mortality events starting roughly 700 years ago.
The authors come to the conclusion that the Black Death was a viral hemorrhagic disease, most probably caused by a filovirus (the same family as Ebola), since the symptoms are a close match for Ebola fever, including the internal organ liquefaction that is the primary killing mechanism of Ebola fever (this was described by several doctors of the time). The main difference in the epidemiology is the much longer latency period of the Black Death, compared to the modern forms like Marburg or Ebola. The modern forms have a sufficiently short latency period that the disease tends to flare up in localized epidemics, but then die down from lack of living hosts before it can spread into a pandemic. Otherwise, the mortality rates are similar to that of the Black Death.
What we now call the Bubonic Plague appears to be an entirely separate disease, that first rose to prominence in the 18th century (1722 Marseilles), half a century after the viral disease last caused a major epidemic, with similar superficial symptoms, but with very different epidemiology.
This is a simplified description of the author's conclusions, but in my opinion they make a quite convincing case.
edit -- I should add that a possible reason that the Black Death caused the nymph nodes to swell and become necrotic could be that it appears that the white blood cells in particular were vulnerable to attack, and the lymph nodes are where several types of white blood cell are produced. The evidence for this conclusion includes the remarkable fact that patients who had been very heavily bled (to the point of fainting) as soon as symptoms started to appear had a much higher survival rate than the general population, perhaps indicating that white blood cells were among the early hosts for the virus, and reducing their number by heavy bleeding reduced the viral load in the bloodstream to a point where survival was more likely.