Black Death
The Black Death was a plague that hit Europe and other countries several times during the medieval period. At one point, the black death reduced the population of eastern Europe by half its previous level.
The most commonly-accepted modern notion is that the black death was a version of bubonic plague, although some academics have recently proposed that the Black Death may have actually been a form of anthrax or an ebola virus variant. Regardless of which virus caused the Black Death, the symptoms were always the same: the formation of painful buboes (swollen lymph nodes) in the armspits and groin, which was the chief method of diagnosis, and a very fast decline in strength and stamina followed by a fatal crash. After death, the bodies of the dead would darken and turn almost entirely brownish-black, which gave the plague its popular name.
Several possible reasons for the spread of the plague were suggested in period, including Jews poisoning the wells, and God punishing mankind.
The real reasons for the rapid spread of the plague more likely included the overcrowding of urban areas, (populations had grown considerably before the onset of the plague), poor sanitation, widespread infestations of vermin such as rats and fleas (the bubonic plague theory suggests that rats carrying the plague also carried fleas, which were the transmission vector to humans), and, possibly, the superstitious notion that cats were evil (thus allowing the rat-populations to flourish in the first place).
The European countryside is dotted with plague-pits (mass-graves) from the many people that died of this horrible disease. Entire towns were wiped out as, once infected, the people had no effective remedy and were doomed to a swift end.
The Black Death was devastating to the feudal system of western Europe. So many died that the stable, established feudal heirarchy effectively ceased to function. In the wake of the Black Death social and geographic mobility reached unprecedented levels, and sowed the seeds for the start of the Rennaissance.
