The Dark Ages were called such for good reason. Not only was the world filled with extreme ignorance and religious anxiety, but the nascent state of the medical world also contributed greatly to the amount of general sickness, pain, and death faced by the masses; the mystifying Black Death causes were in the forefront of everyone's minds, to boot. As a result, it remains one of the most fascinating periods in history - a time when humans were straining to pull themselves from the muck of blindness and into the light of understanding. In this moment, too, some of the explanations that folks came up with to explain phenomena that they didn't understand (like a pandemic) were downright disturbing. The idea that European Jews were to blame for the bubonic plague is one of these frighteningly misguided lines of medical reasoning.
To be fair, one of the lowest points in medieval history occurred in the mid-14th century when the lethal bubonic plague unleashed itself on the populace, wiping out up to 60% of Europe's total population. Fondly referred to as the Black Death, this pandemic was one of the most devastating in human history, beginning in Eurasia, spreading through the Mediterranean, and eventually peaking in Europe between the years 1346–1353. Ripping through vast numbers of people in a short time, the bubonic plague eventually snuffed out an estimated 200 million people.
Aside from human life, the sickness would also severely damage the tenuous state of the European Jews - who had been struggling to overcome centuries of persecution - and lead to widespread violence. As the frightening hand of death swept across the world, it also removed any veneer of religious tolerance and exposed the true sickness within - a disease that many say followed the Jews through the Reformation and into the modern world.
The plague was, indeed, one of the most horrific and gruesome diseases ever to strike the planet. First marked by flu-like symptoms, the Black Death was almost as nefarious as its name. A high fever, chills, and headache would soon lead to black welts and bulges on the skin called buboes, appearing mostly in the lymph node areas of the groin and armpits. Blood and pus would seep from these egg-sized boils, both of which were highly contagious and able to strike a healthy person with very little contact. As the buboes swelled painfully, the stomach flu would ensue, causing the person to suffer from extreme muscle cramps, vomiting, and diarrhea. These grim symptoms were also accompanied by a racking cough, heavy breathing, skin decay, and delirium, all leading to agonizing death in most cases. In some ways, given the lack of medical understanding of the horrific phenomenon, it's little wonder that people thrashed in the dark for outlandish explanations for it.
The outbreak of plague in 1348 brought up a lot of issues for European society, including ones of morality, as people struggled to understand what was happening. They blamed the outbreak on all sort of things - scandalous dress, human depravity, Christian dissension, and of course, the wrath of God. As the disease spread and brutally snuffed out the innocent, people began to cling even more desperately to their religious beliefs as a way to explain the misfortune. Why would God allow this to happen?
Some said the pestilence was the result of widespread corruption in the church, while others suggested it was punishment for the divisiveness of England and France during the Hundred Years' War. After all, Europe had been experiencing infighting for as long as anyone could remember, and massacres, pillaging, and destruction were simply the way of the land. Some saw the plague as the great equalizer that sought to put everyone back on the same level, even if it was subterranean. Others said the Black Death was punishment for the Christians because they did not continue with the Crusades and succeed in pushing the Muslim enemy from the Holy Land. The Crusades of the 14th century had failed - the Muslims still lived in Palestine - and the plague had arrived to remind everyone of this glaring defeat.
As a result of the sickness, many folks took to self-flagellation as a way to appease the Christian God who had clearly forsaken them. By using heavy leather straps studded with sharp metal to whip themselves, these contrite souls hoped to bring about the Lord's forgiveness. Black Death was considered retribution for the sins of humans, like greed, blasphemy, heresy, fornication, and worldliness, and intense penitence was seen as a potential salve. But while the good people of Europe beat themselves senseless as a method of healing, there was an even more pervasive evil that began to take root - the idea that non-Christian Jews were the ones who were truly to blame for the whole affair. In an effort to cope with the terror and uncertainty, people lashed out at their neighbors, while others fretted about the condition of their souls, and the Jews became caught in the middle.
While the connection between Jews and the plague may seem tenuous at best to modern eyes, the Biblical interpretation of their role in Christ's death was still fresh in the medieval mind, especially for the flagellants who believed them to be a scourge on the Christian sensibility. There was no concrete or logical reason behind these beliefs, but as you know, that makes little difference in zealots' reasoning. Fueled by hundreds of years of prejudice and persecution, a simple rumor in 1348 was enough to spark a major blame game, where Jews were suspected of secretly poisoning water sources and "corrupting" the air somehow. As a result, the whole world rose up against them, all congregating around the notion that they alone were the reason behind the plague's horrific hold.