What is it about some people that just gives us the creeps?

A long time friend recently opened a business, and of course I promised to come by and check it out, visit and whatnot. Then I came to learn that his business partner is a guy that I knew a long time ago that scares the daylights out of me. He’s never done anything to me, he’s never been aggressive or violent, but although I haven’t actually seen him in probably 15 years I got a sick feeling in the pit of my stomach at the thought of being in the same room as him.

What is it about people like that, who may not actually do anything, that our gut picks up on? I tend to trust my instincts when it comes to this sort of thing, but why is it that some people set off internal alarms without doing much of anything at all?