The bubonic plague during the Black Death pandemic was primarily spread by fleas, particularly the Oriental rat flea (Xenopsylla cheopis), which were parasites on black rats (Rattus rattus)

. These infected fleas would bite humans, transmitting the bacterium Yersinia pestis , which causes the plague

. The fleas typically lived on rats, and when the rat population died off due to the infection, the fleas sought new hosts, including humans, thereby spreading the disease

. Recent research also suggests that human ectoparasites such as body lice and human fleas may have played a significant role in transmitting the plague during the Black Death, potentially even more so than rat fleas

. This alternative transmission route could explain the rapid spread of the pandemic in Europe. In summary, the main creatures responsible for spreading the bubonic plague during the Black Death were fleas (especially those on black rats) and possibly human ectoparasites like lice