As mentioned, it looks vaguely like some kind of horrible insect or a spider or something. Probably close enough to itch at your subconscious.

I agree. Trychtophobia (spelling) is definitely a real thing, so I don't think it's out of question. It seems that (in some people at least) certain patterns or images can have very, uh, "deterring" properties, I guess.

The biohazard symbol looks flat out dangerous to me. While the radiation trifoil scares me because I've learned what it means, the biohazard symbol is something more instinctual to me. It says "someone who knows more than you do wanted to freak you out by putting this here, so stay away!"

I wonder if it's because it looks like three sets of pincers. I feel like the effect would be muted if the outer circles were consistent width and didn't end in points.

Growing up, I had superimposed the belief that the central circle was supposed to represent an original infected cell, and then, I thought the sharp hooks were supposed to be forming three new infected cells violently emerging from the original infected host, destroying the host in the process.

The imagery seemed to make sense to me, since viruses load up in a cell, and force the cell to churn out copies until it explodes.

I figured the knifey looking sharp hooks were just stylization, to communicate danger and get the point across. (see what I did there?)

Apparently even babies have an innate fear of spiders (and snakes)(according to at least one study).

http://www.livescience.com/2348-fear-snakes.html