The Black Death was a plague that swept through Europe from 1348-1351 and killed an estimate of 25-60% of Europeans. That was about 2/3 of the population (75-200 mil). The Black Death ended in 1351 but continued to return till the fifteenth century.
The Black Death followed a period of population growth in Europe which combined with two years of cold weather and rains that wiped out grain crops. It caused a shortage of food for humans and rats. That caused people and animals to become over crowded in cites, providing disease in the environment.
Many experts agreed that the plague was caused by a pest, a bacteria carried by fleas that live primarily on rats and other rodents that were common in medieval times.Medieval doctors thought there were several causes, but many people thought it was a punishment from god for the sins of the people.
Doctors thought that bad smell could drive out plague so people drank urine and took nasty remedies to get rid of it, but that made it worse. The very rich also tried medicine made of gold and pearls.
Plague still continues to survive in Asia, Russia, Southwest America, and other countries where host rodents and fleas live, but today it is rarely fatal and not very common.