What?! The immune system lies largely (if not entirely) inside he body...not the outside.
Except when fungus grows on your skin and other bacteria enters in your bloodstream in the tiny ventilation holes that are on your skin, then it becomes an internal battle.
Not to mention leprosy and other disgusting things that can happen by not having atleast basic hygene.
Pigs atleast have think skin, and even they get sick sometimes from not having a bath, but humans must wash themselves lol.