Black rats enter Europe carrying fleas with the bubonic plague. These fleas would then bite the people, and transport infected blood into their bodies.
After being infected by the plague, people would rely on Plague doctors to cure them. Most remedies would not work, or would make the patient worse. For example, they would remove the "bad blood" and they would bleed to death. Over 30% of Europe's population died because of the Plague.
After having so many dead bodies and little land to bury them, they would throw dead bodies into the river. This caused the river to be infected and spread to other cities.