We're told that stress can predispose a person to physical deterioration and illnesses. Paranoid thinking is caused, I believe, by some kind of psychic stress, and, then, it creates more psychic stress.

I don't think paranoid delusions about one's spouse cheating are really about fear of being cheated on. I think it's a form of irrational obsessive thoughts. This, IMO, is like the fear of germs that makes a person wash hands 50 times a day, or scrub things beyond reason. You can't reassure this person by going around disinfecting things. No amount of cleanliness is clean enough. Often, the person may very well know the obsession makes no sense. They just can't get rid of it. These are unwanted, intrusive thoughts. I think the best approaches to manage this obsession can be learned from experts in OCD who treat that condition, which is notoriously difficult to treat.

There are physical conditions that can contribute, or even cause, this. I knew a man who was suddenly having obsessive, intrusive, upsetting thoughts. He was being treated by a psychiatrist who was exploring the roots of this thinking. It just made him worse. Turned out that the antibiotics he was getting (a lot of them) were toxic to his brain. The drugs were stopped and his thinking went back to normal. It was an amazing thing to witness.