A multifactorial model of phases in the development and progress of physical illness is described, and the model's utility is illustrated. The model consists of antecedent and concurrent conditions and consequences related to physical, psychological, and social factors and their interaction. The application of the model is illustrated by a theoretical discussion of childhood asthma and by a case study of a young adolescent presenting muscular weakness in which it was extremely difficult to delineate the parts played by physical, psychological, and social factors in the causes of the affliction. (RH)