Poe's word choice is his first paragraph paints a gruesome picture of a horrible, painful, deadly experience known as the "Red Death." Words like devastated, fatal, horror, hideous and victim are not happy words--they are words we associate with terror and death. He then goes on to describe the symptoms and manifestations of the illness in particular detail--mentioning blood, sharp pains, dizziness, bleeding, and the scarlet stains. If this is not enough to terrorize the reader, Poe then says that if a person is afflicted with the disease, other people will shun him or her and not be willing to help.
As readers, we are asking ourselves why Poe is describing such horrible suffering and pain in such detail--it must be essential to the rest of the story for some gruesome reason.
Then, without so much of a transition or explanation, Poe starts his next paragraph with Prince Prospero (prosperous, which those who are afflicted with the illness are not) being happy and wise. Again, we as the reader are left wondering--why did he tell us about the illness? How is that going to come into play in this story?
The juxtaposition of the horrible suffering of the misfortune and the happy-go-lucky prosperous prince is jarring, to say the least.