The medical model proposes that it is useful to think of abnormal behaviour as a disease. Became the conventional way of thinking about abnormal behaviour during the 19th and 20th century and is still the dominant influence today. The medical model allowed for those suffering with mental illness to be viewed with more sympathy and less hatred and fear. Some argue that disease or illness can affect only the body, hence there can be no mental illness: argue that abnormal behaviour involves a deviation from social norms rather than an illness. Diagnosis distinguishing one illness from another. Etiology refers to the apparent causation and developmental history of an illness. Prognosis a forecast about the probable course of an illness. Formal diagnosis is made by mental health professionals. Rely on the following criteria: 1. Their behaviour deviates from what society considers acceptable: 2. Based on a person"s report of great personal distress.