I have lived according to the principles taught in A Course in Miracles for 4 years now. This book seems very strange at first, until you realize that all its teachings are true. How do I know every word is true? By literally applying the lessons to my life and observing what happens.
On the topic of illness, the course teaches that "a sick body does not make any sense." Explaining further, "The ego has a profound investment in sickness. If you are sick, how can you object to the ego’s firm belief that you are not invulnerable?"
"Sickness is a way of demonstrating that you can be hurt. It is a witness to your frailty, your vulnerability, and your extreme need to depend on external guidance. The ego uses this as its best argument for your need for its guidance. It dictates endless prescriptions for avoiding catastrophic outcomes. The Holy Spirit, perfectly aware of the same situation, does not bother to analyze it at all. If data are meaningless there is no point in analyzing them. The function of truth is to collect information that is true. Any way you handle error results in nothing. The more complicated the results become the harder it may be to recognize their nothingness, but it is not necessary to examine all possible outcomes (…) in order to judge them truly."
"The Holy Spirit teaches you to use your body only to reach your brothers, so He can teach His message through you. This will heal them and therefore heal you. Everything used in accordance with its function as the Holy Spirit sees it cannot be sick. Everything used otherwise is. Do not allow the body to be a mirror of a split mind. Do not let it be an image of your own perception of littleness."
"Health is seen as the result of relinquishing all attempts to use the body lovelessly. Health is the beginning of the proper perspective on life under the guidance of the one Teacher Who knows what life is, being the Voice for Life Itself."
Sickness is of the ego and, like everything else of the ego, it is an illusion. Only our sick belief in it makes it seem real. Yes, I recognize that this lessons seems absurd on the surface, but if you choose to become a student of A Course in Miracles, you will undo all fear and sickness to know truth and rest in joy.