I don’t think it “causes” illness or makes one more prone to it, but the way that the person obsesses over their body and always think that they are sick is an illness within itself since it disrupts their lives and makes them unable to function. I know a hypochondriac at my school. I’ve brought it up with her and at first she didn’t take it well (she got defensive) but agreed to seek counsoling. I suggest being gentle with someone like this (don’t say right away that it’s “all in their head”) and just say that you’re worried about them and their wellbeing, and that you think they may have a psychological condition that makes them feel sick a lot. By going in to get this treated, they will have a better life and that’s what you want for them. Then, encourage them to seek professional help.