Challenges to primary care from co- and multi-morbidity.

Medical education is based largely on the challenges of providing high-quality care for specific diseases. Most learning in medical school is carried out by teacher-researchers with expertise in one particular disease or, at best, teachers with special knowledge in one type of disease or an intervention to treat or manage one type of health problem, leading to health care in many countries being led by specialist rather than generalist medicine. Increased effectiveness of health services’ interventions that delay death by managing (although not necessarily curing) diseases has led to a marked increase in the coexistence of separate diseases in individual people. Older literature expressed this notion by the term ‘co-morbidity’: the co-occurrence of unrelated diseases. Total morbidity is not the same as the sum of different diseases – despite the fact that virtually all population data on diseases assumes that it is. The sum of deaths attributed to individual diseases in the world is greater than the total number of deaths (Murray et al., 2004). Neither morbidity nor multi-morbidity is randomly distributed in populations. People and populations differ in their overall vulnerability to illness and resistance to threats to their health; some have more than their share of illness and some have less. Clustering of diseases is a result of a complex pattern of interacting influences, extending far beyond biological vulnerability. It is more common in socially deprived populations and more common in children as compared with its expected frequency based on frequency of individual diseases in populations (despite lower frequencies of morbidity). This morbidity mix (sometimes called ‘case-mix’ by health services managers) is often called multi-morbidity. When considered in the context of demands on health services, it is known as ‘morbidity burden’. Over time, and particularly in the last decade or two, the frequency of diagnosed morbidity has increased, at least partly as a result of lowered thresholds for diagnosis, inclusion of new diagnoses (including some risk factors, such as obesity) and perhaps also as a result of true increases in some diseases (such as those resulting from environmental insults over time). As a result of these changes, the frequency of multi-morbidity is increasing. It is not necessarily the case that increased multi-morbidity would be associated with increased morbidity burden. For example, among the elderly in the United States, the percentage of people with five or more diagnosed conditions who reported being in excellent or good health increased from 10% to 30% between 1987 and 2002 (Thorpe and Howard, 2006). Thus, morbidity has decreased (by self-reports), but physicians are generating more interventions for the diagnosed conditions and, hence, greater burden on the health system. Increases in multi-morbidity are associated with great increases in costs of care, hospitalizations that should be preventable, and adverse events (Wolff et al., 2002). Because it is also associated with increased likelihood of referrals (Forrest et al., 2006), it has great impact on the balance of use of services between primary care and specialist physicians. Although disease-oriented specialists tend to see people with LESS severe disease (Hartz and James, 2006), they dominate the care of people with high burdens of morbidity because of the multiplicity of disease types and, therefore, different types of specialists; this is especially the case in the elderly, at least in the United States (Starfield et al., 2003). Because excessive use of specialist care is unnecessary, potentially dangerous, and very costly (Starfield et al., 2005), consideration of their appropriate use is warranted. Use of specialist services varies widely from place to place, even after controlling for degree of morbidity; in the United Kingdom, it is approximately one-third of that among insured people in health plans in the United States (Forrest et al., 2002). Although much higher than in the United Kingdom, it is lower in both Canada and Spain than in the United States. This is the case both for total number of specialist visits as well as proportion of the population with at least one specialist visit in any given year. Controlled for degree of multi-morbidity, the more different