It originally starts in a rodent. Then the rodent is bitten by a flea, and in turn, the flea bites the human, and the human receives the plague.
The Black Plague, aka the Black Death, was one of the most devastating pandemics in human history. It resulted in the deaths of an estimated 75 to 200 million people and peaking in Europe in the years 1346-1353. The pathogen Yersinia pestis was responsible for the several forms of plague.