First, your body is not you. You created the body as a tool to experience limitation. Since it has form it is also based on limitation. You control your body, not the other way around. Second, realize that anything outside yourself is not real.
Life is based on the infinite and is therefore infinite itself.
Therefore, fear does not exist either. But if I'm sick, I can't force my body to perform like it was healthy. You are again confusing cause with effect. The body is a reflection of your thinking patterns. As long as any amount of fear exists in your thinking patterns, the body will reflect it in the form of ailments or disease.