Poison mob?

I am trying to make a Scorpion mob that can poison the player.  Yet I don't know the best way to do this because the "add potion effect" in entity management, I think would give the potion to the mob & not the player.   I would like the mob to poison the player when the player hits it or when the player collides with it or make a chance it to hit the player giving poison.    I think it would work with a block, to make a poison mine that if you walk on it, I remove the block & add potion effect, that should target the player,  but when it is part of a mob procedure how do you know when it will target the mob & when it will target the player?   because it would be nice if I could give my mob resistance as you might expect a bug to have thicker skin.    So I want a way for the mob to apply  resistance potion to itself  &  poison potion to the player.   Can anyone help or suggest