For a while, psychologists assumed that we’re so afraid of creepy crawlers like spiders and other bugs because of their association with a “disgust” level. Humans actually developed a sense of disgust as a form of self-protection: to recognize slimy, rotten foods or other items like vomit or feces that might be highly-infectious or diseased. Sensitivity to disgusting things, then, helps humans avoid bacteria and sickness.

But why are we so afraid of spiders? It’s usually their creepy-crawliness that gives us the shudders, and makes the hair on our necks stand on edge: not necessarily any fear about them spreading bacteria. But psychologists overall link humans’ sensitivity to disgust to not only diseased things like human fluids or old food, but also “fear-relevant” animals like slugs, rats, bats, snakes, cockroaches — and, of course, spiders. These animals are usually involved in some way in the spread of disease: rats spread the bubonic plague, and bats carrying the Ebola virus are what may have started the epidemic in West African countries.

Of course, there are some poisonous spiders out there — those are certainly dangerous and related to illness. But most spiders are harmless. It’s possible that in the past, humans associated all spiders with poison and illness, thus harboring a fear that was then conditioned generations down the line. Fear of spiders, or arachnophobia, is one of the most common phobias; it is, however, mostly irrational because we’re far more powerful than itsy-bitsy spiders. It you want to overcome your fear of spiders, there’s an app for that: you can condition yourself to playing with cute spiders on your phone before they graduate to more realistic ones.