The bubonic plague, also known as the Black Death flew into Europe clinging to the wings of a blind angel of death in 1347. Best known for its uncontrolled killing through social levels and areas the bubonic plague killed anyone it came into contact with through a variant of ways. The symptoms showed only hours before one's death, as if tolling your last hours. There was no stopping the Black Death.
The streets were unpaved filled with rotting human filth and often made a home by the lower class of the city, including rats. The angel of death in this case was black rats and the fleas they carried. Driven by an unending search for food, the rats had migrated slowly westward along the caravan routes. With the rats had travels the fleas. Within these fleas stomachs were thousands of deadly bacteria that caused the bubonic plague, which only moderately affected the black rats. Sometimes though, the rat would die from the bacteria invading its lungs or nervous system and the flea would be in need of a new host. .
Aiding the flea in its search was its powerful legs, which could jump more that 150 time the creature's length. In most instances the flea landed on another rat, but if not it might land on a human being instead. As soon as it had settled on the human's skin, the flea would begin to feed, and the whole process would start over. Once a person was died however the flea might stay on the clothes and wait to jump on to the next human contact. Because of the way the bacteria traveled the all of the population was at risk.
Europe's population at that time was broken up by feudalism and yet the disease touched all of the social classes. Children, and parent alike were targeted. The only way to really break up the population into higher risk verses lower risk was occupation. Certain professions suffered a higher mortality, especially those whose duties brought them into contact with the sick--doctors and clergy.