The following section focuses on the causes of disease rather than on a detailed description of each entity. It represents one method of classification. There is considerable overlap in categories; certain diseases grouped as metabolic-endocrine in origin could also be classified as diseases of genetic origin. Indeed, the interdependence of the organ systems, the metabolic pathways, and the defence systems renders finite classification in medicine difficult. The human body acts as a unit—an individual—both in health and in disease.