The Black Death, also known as Bubonic plague, started in Central Asia in the early 1300's. The most likely scenario for its spread points to Mongol rulers in Asia who had settled down from their rampages to establish stable caravan routes from China to the Black Sea where Italian merchants would trade for the silks and spices so highly valued in Europe. Ironically, these trade routes were also the invasion routes of a very different sort. Apparently, the Asian black rats, which carry the fleas that carry the plague, burrowed into the caravan's grain sacks and hitched a free ride across Asia. Alien, indeed!
.
This plague had depopulated whole regions of China and India. In 1347 when a Genoese ship pulled into the Sicilian port of Messina with half its crew dead or dying from plague, The Black Death had arrived.

The Plague quickly spread death and terror across Europe, sweeping through Italy in 1347, France in 1348, and the Low Countries, England, and Scandinavia in 1349. Its pattern was to flare up in the summer and taper off in the winter, only to flare up again and sweep onwards the next summer. By 1350, it had pretty well passed on, leaving in its wake a population decimated by its effects. Maybe half of Europe's population was dead, and the effect was huge.
The plague hit Europe six more times by 1450, each time with less severity than before, since more survivors were immune to it. A favorable genetic mutation is still found in their descendants. And those without resistance were weeded out by natural selection. Still, some 30-40% of Europe's population was lost. Census figures in England fell from 3.7 million in 1348 to 2.1 million by 1430. Europe was not free from the Black Death's ravages, suffering recurrent outbreaks until the early 1700's.
.
Why it receded is also a matter of controversy, with such theories as the European brown rat driving out the Asian black rat, tile roofs replacing thatched ones where rats often lived, and the more deadly plague microbe, which more readily killed off its host and left itself no place to go, being replaced by a less deadly version. The third pandemic started in 1850 and lasted until 1970. It arrived in America on a ship about 1905 in San Francisco, and still is found in rodent population pockets around the hills south of the city.

Probably best to look on Wikipedia. It could probably answer all your questions.

Rats on ships spread it. That lead to the death of millions.

rats had the plague .....fleas bit the rats......infected fleas bit people. people moved and so did the plague