Functional problems or illnesses are those that fall into the gray area between optimum health and disease. Pathological illness is the term used to designate when the process involves change in the actual structure of the tissues, such as a fracture, tumor, or tissue-destroying infection.
Functional illness might be compared to a car that is badly in need of a tune-up, but is still running. Pathological illness is present when one of the car’s parts is broken. Left unchecked, the declines in health associated with functional illness may later lead to pathological illness.