The flees would carry the plague,then it got to rats. rats would be seen everywhere people didn't suspense that,that was the reason people were dying.
They would throw infected bodies in the ocean because the pope didn't know what to do with the body. he bought a graveyard but it got full. throwing the bodies in the water made it worse.
It took 10 years for the plague to spread across Europe. The mongols would throw the infected bodies over the wall to the other cities. By the time the citizens escaped they already got infected with the plague. so they were spreading the disease more.