The Black Death is the modern European name for the most severe single epidemic in history. This epidemic of plague lasted from the mid-1340s in central Asia and finally burned out in 1351 in Sweden and Russia.

The plague bacillus, yersinia pestis, is endemic in central Asia and north India, but it has occassionally spread far beyond those boundaries. The reasons for the vast spread of the Black Death are a matter of debate, but it is known that the Caucasus region and the Middle East were hard hit by the plague in 1345 and/or 1346. The plague entered Europe through Mediterannean ports, most notably Genoa, in the fall of 1347.

We now know that fleas and rats served to spread the virus in many areas, although many people undoubtedly caught the disease directly from other humans. It swept through Europe over the next 3 years, killing perhaps 30-40% of the people of Europe. Cities and monasteries often had higher mortality rates and some villages and monasteries were wiped out by the disease in a matter of weeks or months.

The Black Death caused a major revolution in European economics and culture. The massive depopulation led to a rapid rise in wages that shook the foundations of the economic order. Periodic outbreaks of plague continued for the next 350 years, and many people faced with imminent death and the apparent end of the world rejected the existing social and religious order. There is a notable liberalization of social and sexual mores after the Black Death, and European society developed a fascination with death that it never entirely lost. Although there were many signs of coming change in the early 14th century, many have argued that the Black Death was the decisive element in the collapse of the social order of the High Middle Ages.

A note: there is no basis to the "Inquisition caused the plague" story outlined above by Saige. The Roman Inquisition of the Middle Ages was explicitly ordered not to prosecute witches in 1258. The witch hunts are a product of the enlightened Renaissance and not the Middle Ages--the Malleus Maleficarum or "Hammer of Witches," was written in 1484 and the witch-hunting craze only began after the publication and dissemination of that book. There was no mass persecution of witches in the 1340s and so it is impossible that witch-burning could have resulted in the Black Death.