The Black Death was a substantial factor in the collapse of Medieval Society in Europe. Scholars of the time called it in Latin astra mors, meaning "dreadful death." It was caused by a bacillus known as yersenia pestis which was carried by fleas which only lived on Asian Black Rats. The disease is thought to have entered Europe via ships from Asia carrying the rats which are not native to Europe.
The effect of the plague was a slow, lingering death which wiped out as much as one third of Europe's population. Agnolo di Tura, who survived an outbreak, described its effect:
…the victims died almost immediately. They would swell beneath their armpits to their groins, and fall over while talking. Father abandoned child, wife husband, one brother another…And none could be found to bury the dead for money of friendship…And in many places great pits were dug and piled deep with the multitude of the dead. I, Agnolo di Tura buried my five children with my own hands.
A French scientist offered a more dispassionate, even cold description:
All the matter which exuded from their bodies let off an unbearable stench; sweat, excrement, spittle, breath, so fetid as to be overpowering; urine turbid, thick, black, or red.
The plague did not fall as a blanket over Europe; but was more of a cascading wave, passing through a given area over a period of about three to five weeks, then moving on. This actually heightened the panic caused by the plague, as many people could see its movement toward their town or village and could not stop it. Irrational panic resulted from fear of the coming plague; neighbors would not allow others in their homes for fear of transmission; roads entering towns were blocked allowing in only travelling merchants (whose wares probably contained the rats carrying the disease.) Some believed the plague was God's punishment for Europe's sins and went through the streets beating themselves as repentance for the area. These flagellants often quickly determined that the plague was caused by Jews poisoning wells, and slaughtered European Jews wholesale.
Strange remedies were offered, none of which was effective. Among the more bizarre was to breathe the vapors of latrines, as the smell would presumably destroy the miasma which carried the disease. The only true solution to the plague was to move out of town until it passed. Giovanni Boccacio's Decameron is the story of ten young people who left town for ten days for the plague to pass and entertained themselves during that time by telling stories.
Socially, the population of Europe was devastated. Town administrations were wiped out, and there were few artisans or tradesmen. On farms and small villages, peasants and serfs lost prized livestock to the plague and faced starvation. The loss of tradesmen led those left to demand higher wages, and those on farms to leave for work in cities. Both were frustrated. Landlords tied serfs to the land; and wages for tradesmen were frozen. The end result was unrest and in many cases riots and revolts. Peasants had no real weapons, and revolts were put down harshly.