Immediate cause of the black death

It is cause by a bacteria such which is the Yersinia Pestis.

Explanation:

This bacteria is most on rodents such has rats. On that time inside the boats there where a lot of them and whenever they came ashore some left the boats or some went on the boats spreading the disease.

The Black Death is believed to have been the result of plague, an infectious fever caused by the bacterium Yersinia pestis. The disease was likely transmitted from rodents to humans by the bite of infected fleas.