<human>:  What was the cause of the Black Death?
<bot>: The Black Death was a pandemic of bubonic plague that swept through Europe in the 14th century. It killed about half of the European population. The plague was caused by the bacterium Yersinia pestis, which is spread by fleas. The plague was so deadly because it was so contagious. It was spread by fleas that lived on rats, which lived in the same areas as humans.