The Bubonic Plague, more commonly referred to as the "Black Death," ravaged Europe between the years 1347 and 1350. During this short period, 25 million people, one third of Europe's population at the time, were killed. Thousands of people died each week and dead bodies littered the streets. Once a family member had contracted the disease, the entire household was doomed to die. Parents abandoned their children, and parent-less children roamed the streets in search for food. Victims, delirious with pain, often lost their sanity. Life was in total chaos. The Plague was a disaster without a parallel, causing dramatic changes in medieval Europe. Coming out of the East, the Black Death reached the shores of Italy in the spring of 1348 unleashing a rampage of death across Europe unprecedented in recorded history. By the time the epidemic played itself out three years later, anywhere between 25% and 50% of Europe's population had fallen victim to the pestilence. .
Primarily fleas and rats transmitted the Black Death. The stomachs of the fleas were infected with bacteria known as Y. Pestis. The bacteria would block the "throat" of an infected flea so that no blood could reach its stomach, and it grew ravenous since it was starving to death. It would attempt to suck up blood from its victim, only to disgorge it back into its prey's bloodstreams. The blood it injected back, however, was now mixed with Y. Pestis. Infected fleas infected rats in this fashion, and the other fleas infesting those rats were soon infected by their host's blood. They then spread the disease to other rats, from which other fleas were infected, and so on. As their rodent hosts died out, the fleas migrated to the bodies of humans and infected them in the same fashion as they had the rats, and so the plague spread.
In the text, there were several ideas on how the Black Death actually developed. The first document, written by Ibn Khaldun, described the plague in the best detail.