Originally Posted by
IndiReloaded 
^The instinctive response 'making ones skin crawl' to this is probably more to do with a biologic aversion to illness (b/c we don't want to get sick ourselves). We associate bad smells, looks & 'craziness' with poor health.
and I read somewhere that the phisiological response of "skin crawling" was our version of raising hackles. It's very noticable on animals because of their hair, but we humans just feels the teeny little hairs rising on our skin.
Just thought I'd add that tidbit.