Historians have now arrived at a consensus that the deadliest epidemic in medieval and early-modern history began in the Mongol khanates and spread west: the Black Death, or simply “the plague,” of the fourteenth century. The plague devastated the areas it affected, none more so than Europe. That devastation was in large part due to the vulnerability of the European population to disease thanks both to poor harvests and the lack of practical medical knowledge.
A series of bad harvests led to periods of famine in Europe starting in the early fourteenth century. Conditions in some regions were so desperate that peasants reportedly resorted to cannibalism on occasion. When harvests were poor, Europeans not only died outright from famine, but those who survived were left even more vulnerable to epidemics because of weakened immune systems. By the time the plague arrived in 1348, generations of people were malnourished and all the more susceptible to infection as a result.
Medicine was completely ineffective in holding the plague in check. Europeans did not understand contagion – they knew that disease spread, but they had absolutely no idea how to prevent that spread. The prevailing medical theory was that disease was spread by clouds of foul-smelling gases called "miasmas," like those produced by stagnant water and decay. Thus, people sincerely believed that if one could avoid the miasmas (which of course never actually existed), they could avoid sickness. Over the centuries, doctors advocated various techniques that were meant to dispel the miasmas by introducing other odors, including leaving piles of onions on the streets of plague-stricken neighborhoods and, starting in the seventeenth century, wearing masks that resembled the heads of birds, with the “beaks” stuffed with flower petals.
Not surprisingly, given the dearth of medical knowledge, epidemics of all kinds regularly swept across Europe. When harvests failed, the poor often went to the cities in search of some kind of respite, either work or church-based charity. In 1330, for instance, the official population of the northern Italian city of Florence was 100,000, but a full 20,000 were paupers, most of whom had come from the countryside seeking relief. The cities became incubators for epidemics that were even more intense than those that affected the countryside.
Thus, a vulnerable and, in terms of medicine, ignorant population fell victim to the virulence of the Black Death from 1348 to 1351. Historians still debate as to exactly which (identifiable with contemporary medical knowledge) disease or diseases the the Black Death consisted of, but the prevailing theory is that it was bubonic plague. Bubonic plague is transmitted by fleas, both those carried by rats and transmitted to humans, and on fleas exclusive to humans. In the incredibly unsanitary conditions of medieval Europe, there were both rats and fleas everywhere. In turn, many victims of bubonic plague developed the “pneumonic” form of the disease, spread by coughing, which made it both incredibly virulent and lethal (about 90% of those who developed pneumonic plague died).
The theory the Black Death was the bubonic plague runs into the problem that modern outbreaks of bubonic plague do not seem to travel as quickly as did the Black Death, although that almost certainly has much to do with the vastly more effective sanitation and treatment available in the modern era as compared to the medieval setting of the Black Death. One hypothesis is that those with bubonic plague may have caught pneumonia as a secondary infection, and that pneumonia was thus another lethal component of the Black Death. Regardless of whatever disease or combination of diseases the Black Death really was, the effects were devastating.
The plague exploded across Europe starting at the end of the 1340s. All of Southern Europe was affected in 1348; it spread to Central Europe and England by 1349 and Eastern Europe and Scandinavia by 1350. It went on to spread even further and continued to fester until 1351, when it had killed so many people that the survivors had developed a resistance to it. The death toll was astonishing: in the end, the Black Death killed about one-third of the population of Europe in just three years (that is a conservative estimate - some present-day historians have calculated that it was closer to half!) Some cities lost over half of their population; there are even cases of villages where there was only a single survivor. This was an enormous demographic shift in a very short amount of time that had lasting consequences for European society, thanks mostly to the labor shortage that it introduced.
The only somewhat effective response to the Black Death was the implementation of quarantines. The more fast-acting city governments of Europe locked those who had plague symptoms in their homes, often for more than a month, and sometimes whole neighborhoods or districts were placed under quarantine. In the countryside, people refused to travel to larger cities and towns out of fear of infection. Even though quarantines slowed the spread of the plague in some cases, overall they did little but delay it.
More common than practical measures like quarantines, however, was prayer and the search for scapegoats to blame for the devastation. The spiritual reaction to the plague was, among Christian Europeans, to implore God for relief, beg for forgiveness, and to look to outsiders to blame. Building on the murderous anti-Semitism that had begun in earnest during the period of the crusades, Jews were often the victims of this phenomenon. There was a huge spike in anti-Semitic riots during plague outbreaks, as Jews were blamed for somehow bringing the plague (a frequent accusation was that Jews had poisoned wells), and thousands of Jews were massacred as a result.
Religious movements emerged in response to the plague as well, like the Flagellants: groups of penitent who roamed the countryside, villages, and towns whipping themselves and begging God for forgiveness. Many people sincerely believed that the Black Death was the opening salvo of the End Times, since the history of Europe in the fourteenth century so clearly involved both famine and pestilence - two of the four "horsemen" that were to accompany the End Times according to the Bible (the others, war and death, were ever-present as well).
The Black Death ended in 1351, but the plague returned roughly every twenty years in some form. Some cases were as devastating, at least in limited areas, as the Black Death had been. The plague did not stop entirely until the early eighteenth century - to this day it is not clear what brought an end to large-scale plague outbreaks, although one theory is that a species of brown rat that was not as vulnerable to the plague overwhelmed the older black rats that had infested Europe.