Which 2 types of rather small creatures are primarily responsible for having spread the bubonic plague that killed millions of people in Europe and Asia in the 14th century?

The devastating pandemic known as the Black Death swept across Europe and Asia in the 14th century, claiming an estimated 75 to 200 million lives. This horrific disease, a form of bubonic plague, was caused by the bacterium Yersinia pestis. While human-to-human transmission certainly played a role, the initial and primary spread of this deadly pathogen relied on a specific biological chain involving two very common, rather small creatures.

These tiny insects acted as the crucial intermediary, carrying the plague bacterium from infected rodents to humans. They would feed on the blood of small mammals, particularly the black rat, which harbored the disease. Once a rat died from the infection, these parasitic insects would seek a new host, often a nearby human. When they bit a person, they would regurgitate the bacteria into the bloodstream, thus transmitting the disease.

The widespread nature of rats in human settlements, especially in the crowded and unsanitary cities of the medieval period, provided ample opportunity for this transmission cycle to flourish. Coupled with the abundance of their tiny parasitic companions, the conditions were perfect for the rapid and widespread dissemination of the plague, leading to the catastrophic mortality rates observed across continents.