The bubonic plague originated in Asia and spread along the Silk Road into Europe.

It's clear that factors such as war, famine, and drastic changes in the weather contributed to the spread of the bubonic plague in Europe in the Middle Ages.

I do believe that the plague started in China where rats were infected with the plague. Fleas then bit the rats and they too became infected. When trading ships set sail from China they were infested with the rats that carried the infected fleas. When the ships moored in Europe the rats got off the ship and entered the streets of medieval Europe.