It didn't help that I read Poe's "The Masque of the Red Death" when I was 8 or 9.
The closing sentence leaves the most eery feeling and is one of the best and memorable sentences ever written in literature (in my opinion):
"And Darkness and Decay and the Red Death held illimitable dominion over all."
Then I read this book about the legends of Prague which always have the air of perhaps being true to some extent, to have their roots in real places. "The children's plague" was pretty much the scariest story of my younger years. It's the apocalyptical and almost demonic dimensions this plague had that freaked me out. It's the bizarreness of the cures and myths around it.
Last night, they had an amazing documentary on Channel 4 as well, in which they traced back the lives of a few families living in a poor London street in 1665, during the Plague year. That docu reminded me of this book I have read - devoured! - when I was 16. It's by Connie Willis, called "Doomsday Book", some sort of historical science fiction, about a history student from the future time travelling to the middle age in England and accidentally ending up in the year when the Great Plague broke out. The description of life in the middle age is brilliant, and how people deal with the plague. It's a big book, but I promise you, you won't be able to put it down.