Most illness begins with a negative mind.
It’s been proven again and again. Hypochondriacs, people who are convinced that they are sick even though nothing is wrong with them, experience the exact symptoms of the actual illness. For them, the illness is just as real as if their bodies were ravaged by disease. It is also possible to make yourself ill through constant worry and fear of failure, because the mind constantly strives to turn into reality the things we think about most. Protect your mental health with the same care you give your physical body. Just as your body requires healthy, nourishing foods, and a balanced diet, so does your mind. Make sure you feed it plenty of positive thoughts.
From The Napoleon Hill Foundation.