The black plaque was a common name for a disease called Bubonic Plaque is a bacterial infection caused by a variant of Yersinia Pestis. An extremely infectious and dangerous disease, the disease is known for being carried on the fleas or bodies of animals, commonly rodents like Prairie Dogs and in the case of Europe; Rats.
The diseases causes growth of Bubo which are swollen lymph nodes that are swollen and tender to the point that they could break open. Additionally bleeding through orrifices, muscle cramps, delirium, etc... Are all symptoms of the disease.
