Final answer:
The spread of the Black Death was primarily facilitated by trade routes that allowed for disease-carrying fleas on black rats to travel across continents, along with contributing factors such as warfare, ecological disturbances, and overpopulated cities.
Step-by-step explanation:
A major factor that contributed to the spread of the Black Death was the trade routes linking Asia, the Middle East, and Europe. The bubonic plague, caused by the bacterium Yersinia pestis, was carried by fleas living on black rats, which were common on merchant ships. This connectivity via trade allowed for the rapid dissemination of the disease across continents. Additional factors like warfare, ecological stressors, and massive urban populations also played roles in the pandemic's spread. The Mongol siege on the city of Caffa and the subsequent transport of the disease by Genoese ships to European ports like Messina further exacerbated the outbreak. Furthermore, the social turmoil and the diverse climate events preceding the outbreak created ripe conditions for this medieval pandemic to take an extremely heavy toll on the population.