In truth, all diseases or illnesses of the body come from miscreation by the mind. Level confusion, as used in the next passage, refers to mistaking the things in the body as originating in the body, rather than in their true source, the mind. The matter is dead so far as the body is concerned — physical matter is dead and cannot create something that the mind cannot control. The belief that there is something the mind cannot control is an error. In situations where this may seem otherwise, one need only wait patiently — the source of error stemming from the mind will be revealed in due time.
Taken from "A course in miracles"