Yes, it can be. Look at people who suffer depression. One of the symptoms is aches and pains. This is not because they are physically unhealthy. This is the negativity and stress that the mindset produces on the body. I know, i battle depression. What you project is what you feel. If you constantly think bad, you will feel bad, and this can lead to actually having an effect on your immune system etc..
This kind of thinking also prevents people from doing things that are good for them mentally and physically. Not only is your mind making your body ill, the lack of activity feeds this as well. I've battled this kind of thinking all my life and i've always had some sort of physical issue going on.