Unless said illnesses never existed until the doctors crammed the idea into our brains. Who is to say that this isn't all extreme paranoia? You get a supposed professional to tell a person something and they'll practically believe anything.
I saw a show once. Doctor stuffed a handful of candies into a pill capsule and told his patient that it would cure the idea of him having some sort of odd symptom he was experience. The patient took the candy, dosed up on it, and came back for more with claims that it had been working. Interesting, isn't it?