In the middle age what causes the black plague

The plague likely started in Asia and traveled westward along the Silk Road. The disease was carried by fleas that lived on rats.

There are a lot of ways the bulbonic plague spread. Asian trade ships brought it over to Eurasia. Fleas bit the rats, squirrels ate the rats, birds ate the squirrels, birds flew to the places. Another way the bulbonic plague spread was fleas bit a camel, humans ate the camel, infected humans would cough, non infected humans would breathe the air, etc.