I've read a lot of Stephen King and Dean Koontz. Koontz has been great in that he keeps your attention and you never feel like things are slowing down or going off in weird directions. But to be honest, it doesn't scare me. Most of the Stephen King has entertained me more than scaring me, as well. That said, one book that has scared me for life is Stephen King's "IT". It helps if you haven't seen the movie, because your mind can create much better images and interpretations of what is being read, but to this day I can't walk near sewer drains, and I often stare at pictures longer than I should...waiting for them to move. Some of his short stories have had a similar impact on me, but very few - as I said, most of them were just 'cool' and entertaining.
As for the "Hot Zone", I read that, and wasn't really scared. I suppose the big fear factor in that is how close it is to reality. Koontz stuff could happen, for the most part, but he usually ties in enough make believe to where you don't sweat it. King is all about the not-even-possible stuff. If you want things that are possible, or closer to reality, we can probably drum up some recommendations along those lines.
I assume you're going for more 'scare' than 'cool entertainment.' Is this correct?