Final answer:
The spread of the bubonic plague in the 14th century was mainly due to trade routes, notably the Silk Roads and the Mediterranean, and rats on ships which transmitted the disease. The movement of the Mongol armies and climatic factors also played roles in the dissemination of the plague.
Step-by-step explanation:
The spread of the bubonic plague across Eurasia during the 14th century, known as the Black Death, can largely be attributed to the transmission via trade routes and the movement of armies. In particular, the disease was spread through the international shipping and trades of the Silk Roads and the Mediterranean Sea, largely by means of black rats hosting fleas carrying the plague-causing bacterium Yersinia pestis. The Mongol Empire's protection of merchants and the ensuing increase in the movement of people, as well as ecological pressures like famine, played significant roles in facilitating the spread of the disease. These routes and practices allowed stowaway rats to be transported between various ports and cities, rapidly disseminating the plague.
In addition to the role of rats, the climatic crises of the early 14th century may have intensified the outbreak, and the Mongol sieges in Asia are also noted events that contributed to the disease's dissemination. Further consideration of Yersinia pestis' ecological characteristics has broadened historians' understandings of recurrent waves of plague throughout subsequent centuries. The name "Black Death" was inspired by the bodily symptoms. Though it was evident that contagion was responsible for the spread, the biological mechanisms were not understood during that era.