Comorbidity is a term that means having more than one disorder at various times. Concurrent disorders are those that actually occur at the same time. Neither is a strange concept in medicine. The elderly, if lucky, will only suffer from glaucoma and arthritis, the young don't mind if they have myopia and intermittent asthma. Having a disease is not uncommon; having two is not much less common.

Clinicians know this problem well. It is difficult to treat a person with schizophrenia whose psychotic symptoms are sometimes due to the psychosis and sometimes due to drug dependence. Likewise the combination of personality disorder and somatization disorder, or depression and anxiety, or any combination of the major groups of mental disorders produces more disability, makes the prognosis worse, the clinician's task more difficult, and the family's burden greater. Everyone calls for help when people have concurrent disorders. It can be an emergency, however judging things to be important from what forces itself upon our attention is a general type of human error. Our government swings into action following dramatic rural events like floods, and is slow to pay attention to insidious rural phenomena like the gradual loss of productive farmland by rising salinity, even though the burden of salinity is much greater than the burden of floods. So it is with comorbidity and concurrent disorders. Those that cause alarm will receive help; those that quietly destroy a person's productivity will often be ignored.

The prevalence of comorbidity is often addressed on a disorder by disorder basis. Rates of substance use disorders are examined among those with psychosis (Mueser et al., 1990; Regier et al., 1990), and rates of depression among those with panic disorder (Kessler et al., 1998). If groups of disorders are taken into account, the focus is usually on the co-occurrence of mental disorders with substance use disorders — and the prevalence of one group among cases of the other is not insubstantial, often around 45–55% (Kessler et al., 1997; Kessler et al., 1996; Regier et al., 1990; Ross, 1995). However, in order to put such figures in context it is necessary to examine the prevalence and patterns of all comorbidities in the community, not merely those that come to the attention of health services. Data from population samples allows us to do this. The current chapter takes a population approach to comorbidity, firstly reviewing the findings on the patterns and prevalences of comorbidity from several major population surveys, including the Australian National Survey of Mental Health and Wellbeing (NSMHWB), restricting the review in general to large-scale nationally representative samples. It then uses data from the Australian survey in which a random sample of Australian adults could have their say about what disorder troubled them the most, to estimate the clinical significance of each group of mental disorders. The discussion focuses on the health service planning implications of taking a population approach to comorbidity.