There’s no special reason why the plague that began in 1347 is called the Black Death. It wasn’t called so at the time; the nickname seems to have begun several centuries later, perhaps as a mistranslation. When survivors looked back on those dreadful three years, they called it the Great Mortality.
Like all plagues of the ancient world, the infection made several body systems break down at once. The most visible sign of the infection was the “bubo,” an egg-shaped swelling in the lymph nodes. We named it Bubonic Plague for these painful outbreaks. There were some survivors of the infection, including a few who lanced and drained their buboes. The buboes were ugly but not the most virulent part of the disease.
Bubonic Plague is famous for being spread by rat fleas; this is true, but it seems to have been more strictly true of later outbreaks, including one in the late 19th century when the bacterium was isolated and named Yersinia pestis. In later outbreaks, such as in India, people saw many dead rats. Dead rats are missing from medieval accounts, although other dead animals, such as pigs and cattle, are described. There may have been dead rats, but they were not dying in conspicuous enough numbers.
Yersinia pestis originated in the Central Asian marmot population: the woodchucks of the Silk Road. Global cooling may have flushed them out of northern plains, closer to the Silk Road, and greater merchant traffic meant that more unwary strangers were skinning and eating them—and picking up their fleas. But there’s no way to account for the way Y. pestis spread in Europe unless fleas on human hosts began to carry it. Not only did this zoonotic disease jump from marmots to humans, it moved from marmot/rat fleas to human fleas. It may also have gone airborne for a while. It appears to have mutated several times in the process.
The infection spread along the Silk Road in both directions; it devastated India and China, although Europe was not aware. It reached the Tatars as they laid siege to a Genoese colony on the Black Sea; as they grew desperate, the besiegers catapulted dead bodies over the city wall, spreading infection. Survivors ran to their remaining ships and put out to sea, hoping to escape to other Genoese colonies. But each time they tried to land, the harbor workers saw that they had plague and drove them off. Unfortunately, there was just enough contact each time that every Genoese colony took infection. By the time the remaining ships with the last dying people reached Marseille and Genoa, the infection had begun to take off in all of these port towns. Genoa heard of it in advance and drove off the ships with flaming arrows. The plague still spread (perhaps even from the messengers who brought warning).
Like a newspaper lit in several places, the Mediterranean coast spread the infection from town to village and from boat to boat. Within two months, most port towns had the plague.