The Black Death was a plague that hit Europe and other countries several times during the period the SCA covers. 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 (though I've heard there is some controversy about this, and possibilites that there were multiple illnesses involved).
Characteristics of the plague included the buboes or swollen lymph nodes - the most easily noticeable ones being those near the armpits, and the way a body turned black after death (not sure why) - which gave the sickness its name.
Several possible reasons for the spread of the plague were suggested at the time:
- Jews poisoning the wells.
- God punishing man.
While the real reasons include:
- Overcrowding of cities (populations were booming)
- Poor sanitation (what had been acceptable for small populations didn't scale up to the new cities)
- Fleas carrying the virus from host to host - mainly from the rat infestations which flourished in the overcrowded, polluted cities...
- Silly superstitious notions that cats were evil... thus allowing the rat-populations to flourish in the first place.
The plague was generally spread by fleas from ill/dying rats - if the rats were killed, it just meant more fleas looking for new hosts (ie humans). tghe only way to control the problem was to not allow the rat population to grow in the first place - which meant sanitation and better city-planning.
Needless to say, the European coutriside is dotted with plague-pits from the many people that died of this horrible disease. Entire towns were wiped out as, once infected, the peopole had no effective remedy and were doomed to a swift end.