The difference between the fictional disease, the red death, that Poe creates in this story, and the actual medieval disease that he took as his inspiration for the red death, the black death, is shown in the opening paragraph of the story where the narrator describes the red death and the symptoms that accompany it. Note the following description:
Blood was its Avatar and its seal --the redness and the horror of blood. There were sharp pains, and sudden dizziness, and then profuse bleeding at the pores, with dissolution. The scarlet stains upon the body and especially upon the face of the victim, were the pest ban which shut him out from the aid and from the sympathy of his fellow-men. And the whole seizure, progress and termination of the disease, were the incidents of half an hour.
The red death therefore is a disease that is associated intimately with blood, so much so that the narrator says that blood was the "Avatar and... seal" of the read death, with the major symptom of this disease being "profuse bleeding." In addition, the red death is shown to be so virulent that the course of this disease, right through to death, occurs in only half an hour. In reality, the black death involved black swellings and the speed of the disease was not as rapid. Of course, another major difference is that Poe creates his disease to show that death cannot be outran, no matter how rich and wealthy somebody may be.