What were the social effects of repeated attacks of plauge and disease in the Fourteenth Century?
I must voice my disagreement with the answer posted above, and therefore have elected to add an additional answer.
Hysteria was a factor in the plague, but not a major factor. Hysteria only lasted when the onset of the plague was in sight. It did not descend over all of Europe completely at one time, but rather cascaded and normally left given areas after a few weeks. Death was not random or inexplicable; it was a terror that could be seen coming weeks, even months before it arrived. nor were people ruled by "unbridled passions...in various ways." There were some instances of anti-Semitic attacks, mostly by zealous flagellants who accused the Jews of poisoning wells; but again this was not a "major factor."
The major social effect was a tremendous decline in the population. Over one third of the population of Europe died during the great Plague outbreak of the 14th century. As a result, there were fewer serfs to work the fields, fewer workmen in the cities, even fewer city administrators to maintain order. In the end, rather than hysteria there was a rather sad resignation to the reality of death, as evidenced by the following eyewitness account (emphasis added in bold):
The mortality in Siena began in May. It was a cruel and horrible thing. . . . It seemed that almost everyone became stupefied seeing the pain. It is impossible for the human tongue to recount the awful truth. Indeed, one who did not see such horribleness can be called blessed. The victims died almost immediately. They would swell beneath the armpits and in the groin, and fall over while talking. Father abandoned child, wife husband, one brother another; for this illness seemed to strike through breath and sight. And so they died. None could be found to bury the dead for money or friendship. Members of a household brought their dead to a ditch as best they could, without priest, without divine offices. In many places in Siena great pits were dug and piled deep with the multitude of dead. And they died by the hundreds, both day and night, and all were thrown in those ditches and covered with earth. And as soon as those ditches were filled, more were dug. I, Agnolo di Tura . . . buried my five children with my own hands. . . . And so many died that all believed it was the end of the world."
With the loss of so many workmen, those who remained, seeing that they were in short supply, demanded more wages, but were rebuffed. Serfs attempted to leave manors to seek work in the cities where they could earn decent wages, but found themselves tied to the land. The end result was revolts and rebellions, which were put down harshly and caused even greater loss of life among the peasantry.
With the death of so much of the population, the entire social structure of Medieval Europe collapsed. The rather stable system of "those who pray, those who fight, and those who work," no longer worked. The plague was then a major factor in the demise of the middle ages and the birth of the modern age as the population slowly recovered. An excellent description of the plague as seen through the eyes of common people is John Hatcher's The Black Death: A Personal History. It offers fascinating but often chilling detail.
The major social impact of repeated attacks of plagues such as the Black Death was that these attacks caused mass outbreaks of hysteria and extreme reactions to the psychological stresses of having death seem to strike randomly and inexplicably.
May of these outbreaks of hysteria were religious in nature. People sought to understand what was going on or to try to end the attacks of plague. Some of these efforts were harmful only to the people themselves. An example of this was the flagellant movement in which people felt that they could purge their sins by whipping themselves unmercifully. This, they hoped, would spare them from becoming victims of the plagues.
Sadly, some of these efforts were much more sinister. For example, the attacks of plague were a major cause of anti-Semitism. There were many instances of Jews being blamed as people tried desperately to understand and control what was happening. These outbreaks of anti-Semitism often lead to terrible episodes of violence against Jews.
Overall, the plagues ate away at the fabric of society and allowed these sorts of unbridled passions to rule people in various ways.