The Black Death originated in Central Asia, carried to Crimea by Mongol warriors, and ended up in Europe. The plague was carried by rats carrying fleas infected with a familiar bacteria: Yersinia pestis. Remember the Plague of Justinian? There was only the bubonic variant of the plague present in that pandemic. During the Black Death, it is presumed that all 3 types of plague were present. This created a catastrophe in both cultural and governmental circles. Because up to 2/3 of those infected by the Black Death died, a massive population shortage came about, leading to massive reforms, such as the end to serfdom. While the Black Death killed 25-50 million people in Europe alone, it is estimated that the Black Death killed up to a third of the world's population at the time, between 75 and 200 million people.
A common theme with the Black Death outbreaks is poor hygiene and a lack of medical knowledge. Because people were not knowledgable on personal hygiene routines, coupled with the fact that citizens fled the area of a plague case due to mass panic which further spread the contagion, made for a monster of a disease.