Black pox is a symptom of smallpox that is caused by bleeding under the skin which makes the skin look charred or black. This symptom usually occurs if a patient with smallpox is going to die.

A doctor who encoutered black pox stated that "Doctors separate black pox into two forms — flat smallpox and hemorrhagic smallpox. In a case of flat smallpox, the skin remains smooth and doesn't pustulate, but it darkens until it looks charred, and it can slip off the body in sheets. In hemorrhagic smallpox, black, unclotted blood oozes or runs from the mouth and other body orifices. Black pox is close to a hundred percent fatal. If any sign of it appears in the body, the victim will almost certainly die. In the bloody cases, the virus destroys the linings of the throat. The stomach, the intestines, the rectum, and the vagina, and these membranes disintegrate. Fatal smallpox can destroy the body's entire skin — both the exterior skin and the interior skin that lines the passages of the body."