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. Most likely, however, the Black Death was a series of related viruses with similar symptomologies.
Regardless of which virus caused the Black Death, the symptoms were usually 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. Just before death, the body of the victim would would darken and turn almost entirely purple-black, which gave the plague its popular name; this was caused by the coagulation of the blood inside the veins and arteries of the body, and once such a colour-change occurred there was no recovery.
The real reasons for the rapid spread of the plague most 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 from the Black Death 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 hierarchy 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 Renaissance.