I think most illnesses can. They can also be altogether prevented by eating right. I mean, you look at the major illnesses affecting the public and pretty much all of them can be prevented if not reversed simply by diet. Sugar seems to be the cause of most of the problems.
It definitely plays a huge role. Even if it doesn't prevent or cure illness, being healthy with a strong immune system can't hurt.
I've noticed this. If you look at the most common killers of Americans, the top 5 are largely behavior-centric. Heart disease-- lack of exercise, smoking, poor diet Cancer- the most common lethal type being lung cancer, which is often caused by smoking Lower Respiratory Illness (emphysema, chronic bronchitis)--can be caused by smoking Strokes--poor diet, obesity, smoking Most types of cancer become more common in smokers, heavy drinkers, or the obese.