What is it about slugs?

Specifically, what is it about stepping on a slug in our bare feet that causes us to recoil?

Physically, we are no danger, aside from the small possibility of slipping and falling. We are not expressing concern over the fate of the slug, indeed it probably barely registers to us. We are concerned with the sensation.

Why do we find it unpleasant?

Is this a learned opinion, or something hard-wired? Would a baby crawling along the ground burst into tears if he crawled over a slug, or would he be curious? Or would he not notice at all and just keep crawling?

If it is an evolved response, why? Why is it beneficial to find a squishy, sticky goo “icky”?