The Masque of Red Death is a largely metaphysical character from Edgar Allan Poe's short story known as The Masque of the Red Death (originally printed as The Mask of the Red Death: A Fantasy). It was first published May 1842 in Graham's Magazine and has since been adapted and/or alluded to in many different forms of media.
The disease of the Red Death itself is purely fictional. Over the years, it's been believed to have been inspired by various different diseases, including tuberculosis, cholera, and the Bubonic Plaque (better known as the Black Death). In the story, the disease is described by Poe as causing "sharp pains, and sudden dizziness, and then profuse bleeding at the pores" leading to death within half an hour. Some interpretations have also considered the Red Death to be not a disease, but instead an allegory for the inevitability of death.