What caused the black plaque?

The Black Death (a.k.a the Black Plague or Pestilence) was a deadly 14th-century pandemic believed to have killed about 75-200 million people (30-60% of Europe's population at the time), peaking between 1347 and 1352.

Cause: Yersinia pestis bacterium, usually found in wild animals like rodents. Bacteria were likely transmitted to fleas found on rodents and then by humans who were bitten by the infected fleas.