In the 1330s, a terrifying new disease was recorded in China, where it killed roughly two-thirds of the population. It made its way slowly along the trade routes across Asia and the Middle East, arriving in Europe in 1347; by 1348, it was decimating France and England, where peasants had already been weakened by famine and poverty. The disease came on suddenly, mysteriously, and killed within days; in places, it killed well over half the inhabitants of a neighborhood or village, and some villages were emptied by deaths and by people fleeing the disease (usually transporting the disease to their refuge, spreading it quickly). The disease, now known as the plague or Black Death, was called the great mortality at the time.
After a year or two, the plague died down, only to return in waves about every decade; by 1400, the population of Europe and the Middle East had been reduced by one-third. Population losses continued until 1420 and the population didn’t start growing again until after 1470. The plague disappeared in 1352, only to reappear in 1361 and roughly every decade after that for a century or more; although it is rare now, cases of bubonic plague still show up, and a few people die from it, every year.
Medieval cities were already unhealthy places to live before the plague arrived, due to crowding and a complete lack of sanitation which spread disease quickly once it started; death rates were high and city populations were maintained only by immigration from rural areas. The plague hit cities harder than the country, with up to 50% mortality in most of them. Poorer people were hit harder than the wealthy because they lived in the dirtier, more crowded parts of the cities and didn’t have the resources to leave the cities early on, as the rich did (See The Decameron for an example of how wealthy Italians sat out the plague). Rural areas were hit more sporadically, but when the plague arrived, it often emptied the village. Monasteries, which would seem to be safer due to their often rural locations, were hit hard because the monks had always cared for the sick.
No one knew what caused the plague; it was usually considered to be the wrath of God on a sinful population, although some people blamed it on the Jews poisoning wells and Jews were persecuted across Europe. In reality, it was carried on black rats which traveled on ships from one port to another. The rats carried fleas which then jumped to humans; the plague needed both carriers (human and rat) to complete its life cycle. [Or at least that is the traditional theory; modern medical research is raising doubts about that, since the plague struck in places without rats (Iceland) or too hot for fleas (the Mediterranean). More recent candidates for the Black Death are anthrax and Ebola, but the debate is still in play.]
The plague had substantial social impacts. In the short run, many families broke up under the weight of the fear and hysteria caused by the plague. People turned to prayer and gave land to the Church in order to “buy” God’s protection from the Black Death; at the same time, other people lost faith in God and became party animals, chosing to live for the moment rather than the afterlife. In large measure because of the unpredictability of the plague, a morbid fascination with death spread through society; a favorite artistic motif was the Dance of Death, which showed Death leading a line of people from all walks of life.
Longer term, the increase in available food and resources eventually meant healthier people. There were fewer rural laborers to work the land, which meant that they could command a higher wage for their work; governments tried to counter the rising wages by fixing the wages and tying laborers to the land, but the efforts collapsed. Peasants not only found better jobs in the country, they moved to town in search of a better life. As peasants became more mobile, the feudal system collapsed; this mobility may be the cause of the Great Vowel Shift (when vowels migrated from one part of the mouth to another), the main reason that the spelling of English words no longer reflect their pronunciation.
Although the church received many donations during the plague, the inability of the clergy to contain or prevent plague deaths caused people to doubt or disbelieve in them. Many experienced monks and other church men died while nursing the sick during the plague, and they were often replaced with inexperienced men who had too little training and vocation. Not surprisingly, this led to bad behavior on the part of the clergy and increased distrust on the part of the laity (non-clergy).
Over all, the social dislocations in the century following the Black Death led to a decrease in the power of authority. Medieval medicine didn't work, so doctors began to explore the human body and accept the evidence of their senses rather than accepting what the ancient authorities said even when it contradicted experience; this was the beginning of modern medicine and science. Peasants freed themselves of their lords and were able to search out better opportunities, for the first time in centuries. And the church lost much of its automatic hold over people as they began to question its ability to protect them from death. The accumulation of these changes tore down the medieval systems and opened the door for the Renaissance and Reformation that followed.