Final Answer:
The spread of the bubonic plague across Eurasia was primarily facilitated by the movement of fleas infesting rats aboard merchant ships along trade routes.
Step-by-step explanation:
The bubonic plague, caused by the bacterium Yersinia pestis, spread across Eurasia during the 14th century. The key factor behind its dissemination was the movement of fleas infesting rats. Rats served as carriers of the disease, and the fleas that fed on infected rats became vectors for transmitting the bacteria to humans.
This transmission occurred most notably through trade routes, particularly those involving merchant ships. As ships traveled from one port to another, rats aboard carried infected fleas, which then bit humans at various stops along the route. The interconnectedness of trade networks during this time facilitated the rapid and widespread transmission of the plague.
The Silk Road and other trade routes played a crucial role in the plague's diffusion. As merchants and traders exchanged goods, they unknowingly transported infected rodents and fleas, inadvertently spreading the disease across vast regions. Additionally, the crowded and unsanitary conditions in cities further exacerbated the transmission, providing an ideal environment for the proliferation of both rats and fleas. The convergence of these factors, driven by the movement of people and goods, created a perfect storm for the rapid and devastating spread of the bubonic plague across Eurasia during the 14th century.