By rats and I'm being serious it was from the rats and the fleas biting the infected rats then the fleas jumping on people and biting them that's how it spreadesb
Explanation: The origin of the bubonic plague, also known as The Black Death, was apparently in the dry plains of Central Asia, then it spreaded along the Silk Road, arriving Crimea by the year 1343. From Asia, it was most likely to be spreaded throughout the Mediterranean Basin and Europe by the black rats' fleas that traveled on all merchant ships.
