Final answer:
The Black Death was spread by rats that carried fleas infected with the bacterium Yersinia pestis, which was facilitated by trade routes and movement of people.
Step-by-step explanation:
The Black Death was spread by rats infested with fleas carrying the deadly bacterium Yersinia pestis. These rats were typical stowaways on trade caravans and merchant ships, which facilitated the rapid spread of the plague along major trade routes such as the Silk Road and throughout the Mediterranean.
Increased movement of people during the Pax Mongolica and due to famine and environmental change also contributed to the proliferation of the disease. It's also important to note that while rats were the initial carriers, the fleas would seek human hosts once their rodent hosts had died, thus further spreading the plague to humans.