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):

readability="18">

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.