Various associations have been reported in the literature between severe mental disorders, particularly schizophrenia, and other diseases; the demonstrated association of two (or more) diseases would be of great theoretical and practical importance. If two (or more) diseases occur in the same individual significantly more often than would be expected by chance, the same aetiological factors may be involved, or one disease may be pathogenic to the other, or one condition may be a hitherto unrecognised manifestation of the other, or the patient — as a sequela of one disease — may be exposed to risk factors for the other. In addition to the scientific importance of such discoveries, knowledge of this kind may be useful for the furtherance of public-health programmes, for the organisation of services, and for the prevention of illness.
