The mummy is a corpse embalmed and preserved in wrappings, a monster drawn from ancient Egyptian burial custom and the lore of the "pharaoh’s curse." In tabletop role-playing it is depicted as a tomb-guardian undead that inflicts a flesh-rotting disease (mummy rot) on those it touches.
The idea that a thing meant to stay sealed breaks loose and passes sickness to the living is horror’s old grammar of the price for touching what must not be touched.