how do i make my mob effects work right?

i want to make a thing where, if you get hit by the mob, it gives you an effect, i know, so original, but the main problem with that is that it works to well, meaning, if i get hurt, AT ALL, then i take that effect if im near that mob, is there something that im missing that i havent seen before? maybe im missing a "if x mob hits x entity" procedure? im not sure, but taking fall damage and then going blind is not what id call fun. so please help me out here