What scares you the most?

Well, I like writing. I've been wanting to write a good horror story for a while now. Problem is, I think it's one of the most difficult genres to write about, at least for me. You need to capture the reader's imagination and fear, tie them together, and manipulate them to feel whatever you want them to.

So, background aside, I need a "monster" for the story I wish to write. Not so much in a literal sense, but I need a basis for the ground fear in the story. It can be psychological, supernatural or physical. I figured the best way to create said "monster" would be to ask the masses. So what would make you check over your shoulder? What would make you hesitate when walking into a dark hallway? Try not to be limited to the terms of human reality. I need beyond that. Basically, I need you to help me scare the crap out of you. Kind of contradictory I know. Consider it a literary experiment! :)

Thanks!

Ooo. Psychosis and paranoia. What's real and what's not? That would be a good one to work with. More so for a good reading story though. I don't know if that would relate too well with the reader.

I may do it just to say I did.

Story: The boogeyman snuck up behind the girl and dropped worms into her newly acquired red Hollister shirt!

Sporx: OMIGODOMIGODOMIGODOMIGOD!!!!!!!!

Sorry, one of those funny mental images.