Illness is an unpleasant experience that deviated from the normal state of health.
The concept of illness behavior provides a useful way of understanding and describing the many psychosocial influences that affect how people monitor their bodies, define and interpret their symptoms, come to view themselves as sick and disabled, take remedial action, and use lay and professional sources of help (Mechanic, 1978). The concept draws on psychological theories of perception, cognition, and meaning attribution and on theories of social relationships. A crucial premise in the study of illness behavior is that illness, as well as the illness experience, are shaped by psychological, social, and cultural factors irrespective of the genetic, physiological, or other biological bases of disease. Hence, as used here, illness and disease are distinct. Disease refers to a specific clinical entity characterized by a disturbance in the structure or function of any part, organ, or system of the body. Illness is a broader concept that refers to any condition that a person views as sickness (Eisenberg, 1977). Whereas the magnitude, severity, persistence, and character of symptoms affect and establish limits for personal and social definitions of illness, there is considerable variability in what is perceived, how it is defined, the interventions that are considered and used, and the outcomes of illness. Thus, illness behavior is a dynamic response to changing bodily sensations. It reflects not only the individual's psychological predisposition, but also the broader socioeconomic and cultural context within which the individual lives. For the physician, as well as the disability examiner and the law courts, much of what is involved in understanding a person's complaints of chronic disabling pain that seem disproportionate to objective clinical findings may be clarified by the concept of illness behavior. This concept also helps to explain why some people who have well-substantiated diagnoses of painful diseases and obvious physical limitations continue their usual routines and push themselves to perform beyond the levels observers might expect.