Illness can be a synonym for disease or it can be a person's perception of having poor health. Disease is an actual physical, pathophysiological process which can cause an abnormal condition of the body or mind. Illness and disease are not necessarily the same. Most people who have a disease will feel they have an illness, while others will feel perfectly healthy. A third group (although small) may claim illness although they do not actually have a disease.

People could have dangerously high blood pressure (hypertension) and be in danger of having a fatal heart attack or stroke, yet they feel perfectly well and believe they do not have an illness.

The biopsychosocial model draws a distinction between the actual pathological processes that cause disease, and the patient's perception of their health and the effects on it, called the illness.