Why are we so scared of bugs?

I don’t mean that in a “who the hell would be scared of a bug?” type of way, since I am completely phobic of cockroaches, terrified of junebugs and freaked out by spiders, but what is it that makes us so scared of these little tiny creatures? I was wondering this today, since now that we have our windows open there’s a billion of the little fuckers wonderful beings wandering around my house, and while I was doing dishes some random bug landed on my hand and I screamed like crazy and thrashed around like I was having a seizure, and then I wondered, why? What exactly is so scary about them?