In "The Masque of the Red Death," why does the masked figure's presence cause such a sensation?

Prince Prospero and his thousand friends had quarantined themselves against the Red Death inside an abbey.  After six months of being locked up with the Red Death outside, Prospero decides to throw a lavish masque to celebrate their "escape" from death.  He has a tendancy to like lavish, grotesque costumes, but when someone shows up dressed as the Red Death, he becomes enraged.  His anger stems from his (and the others') fear of the Red Death.  If anyone catches this disease, he/she will bleed from all pores and die within thirty minutes.  No one thought it was funny that someone would dare to mock their fear by coming to the party dressed as the Red Death.  What they failed to realize was that no one can escape death; it comes to us all.