most believe it was due to Yersinia pestis and probably arose in 1334 in the Chinese province of Hubei and appears to have spread to Europe along the Mongol trading routes, arriving in Constantinople in 1347.
it was next reported in Caffa, a trading city in the Crimean Peninsula where Mongols were attacking the city but their numbers dropped dramatically as the Black Death raged through their troops. The Mongols used the infected corpses of their soldiers as biological weapons by catapulting them over the walls of the city.
the Genoese inhabitants of Caffa fled back to their home ports in the south of Europe taking the disease with them. The ships returned with most or all of the crew dead, limping into ports or being washed up on the shores.
by 1347-48, it had spread to Genoa and Venice, by June 1348 it reached England, France, Spain and Portugal.
in 1348-49 it devastated the Middle East, Antioch, Mecca & Mosul.
by 1350 it reached Scandinavia and in 1351, north-western Russia and Yemen.
it continued to return to Europe until the last half of the 17th century, with each outbreak being less lethal.