Single spies and battalions: the clinical epidemiology of mental disorders.

Clinical epidemiology, a term that has been variously defined, is used here to refer to a discipline which, commencing with examination and diagnosis of the individual patient who presents in medical practice, proceeds to study the occurrence of similar, possibly connected cases in the local community, and in so doing may provide hypotheses for population-based studies of disease and its risk factors. While the relevance of this discipline to the modern practice of clinical psychiatry remains largely unexplored, its importance in the search for causes of mental disorder is attested by many instances, both historical and more recent, in which the spread or clustering of psychiatric syndromes in populations could be related to nutritional deficiency, infectious disease, the presence of environmental neurotoxins, the social communication of psychopathology or the transmission of abnormal, harmful behaviour patterns within family groups. Observations made in clinical practice have repeatedly served as the starting point for epidemiological investigation of mental disorders, while conversely epidemiological findings have influenced clinical thinking about their classification, diagnosis, prognosis and morbid risk. A review of the relevant literature underlines the need for a keener awareness of environmental risk factors and a fundamentally epidemiological frame of reference in trying to grapple with the aetiological problems of mental disorder.