Final answer:
The Black Death was a devastating pandemic in the Middle Ages, caused by the bacterium Yersinia pestis, leading to massive fatalities and significantly altering societal structures in affected regions.
Step-by-step explanation:
The Black Death, a recurrent epidemic disease during the Middle Ages, is called a pandemic. It was an outbreak of plague primarily caused by the bacterium Yersinia pestis and was one of the most devastating pandemics in human history. The first outbreak, known as the second major pandemic, occurred in the 14th century, leading to massive fatalities and causing profound social, economic, and demographic changes in Eurasia and Northern Africa. An estimated 25 million people died in Europe and at least another 25 million elsewhere. The Black Death acted in three distinctive forms: the bubonic, septicemic, and pneumonic varieties, with mortality rates exceedingly high. The pandemic made frequent returns, contributing to the end of Europe's Dark Ages and the feudal system by the 15th century.