What was one of the primary reasons for spread bubonic plague?

One primary reason for the spread of bubonic plague was fleas that infested rats and other rodents.

During the epidemic, rats infected with the plague bacterium (Yersinia pestis) carried the fleas and transmitted the disease. The fleas would feed on the infected rats and then pass on the bacteria to other animals and humans by biting them. The fleas acted as vectors, enabling the rapid transmission of the disease from one host to another. The transmission occurred when an infected flea bit a person, introducing the bacteria into their bloodstream.