1.3 Definitions of health and disease

This learning object aims to examine definitions of health and disease and the internationally accepted schemes for their measurement. It explains why these concepts are subject to continual refinement and explores some of the particular definitional challenges facing the health analyst.

Western scientific medicine sees disease as biological fact, amenable to identification through appropriate medical tests. The history of western medicine is frequently described as a series of advances or breakthroughs, as the causal mechanisms of successive diseases are identified and corresponding pharmaceutical or surgical solutions developed. By contrast, although we might consider disease as an abnormal biological condition, the concept of normality is not scientifically defined in most societies. What is considered normal or abnormal is as much a societal, moral judgement as the result of scientific investigation and diagnosis. This means that it is also subject to continual change. For example obesity, a condition that was historically regarded as simply a consequence of particular lifestyle choices, is currently undergoing ‘medicalization’ and becoming the subject of direct medical intervention and drug therapy.

In order that we are able to analyse and compare patterns of health and disease over time and between places, it is necessary that common definitions and standards be used for their measurement. With continual evolution of both medical and social understandings of health, this is not straightforward requirement. At the most basic level, we can recognise the World Health Organization’s definition of health, which has not been altered since its declaration in the 1940s:

Health is a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity

(World Health Organization, 1948)

 

This definition is extremely widely used. Health is here partially defined by the absence of disease, but also implies a degree of wholeness and fulfilment which has implications far broader than the usual scope of medical diagnosis. The practical realisation of this definition of health will therefore vary between societies and individuals and at different stages in an individual’s life. It is related to social factors such as ability to engage in work and leisure activities, and encompasses relationships with others. This latter point suggests some element of comparability: an individual’s perception of their own health (and thus their likelihood of seeking medical assistance) are to a large degree determined by the health of others around them, rather than reference to universally applicable standards.

The health care system does not usually provide us with measures of ‘health’ but rather with diagnoses of disease and it is therefore the recording of disease statistics that has been standardised for the purpose of international reporting. The first International List of Causes of Death was adopted in 1893 and has been periodically revised in order to take into account continual changes in the diagnosis and classification of disease. The most recent version of the International Classification of Diseases is revision 10 (ICD-10), which is used primarily to apply standard codes to mortality (death) data. An indication of the need for change is that HIV and AIDS were unknown as causes of death when the previous revision (ICD-9) was introduced in 1979. A variant of this classification system, the International Classification of Diseases, Clinical Modification, is a US adaptation of ICD-9 used to code morbidity (illness) data captured from hospital and health service records. There is also an International Classification of Functioning Disability and Health, intended to provide a scheme for coding how people live with their health conditions.
 

 
Rooney and Smith (2000) outline some of the difficulties that occur when moving a national recording system from one version to another, including examples of cases where the underlying causes of death are recorded differently in revisions 9 and 10 of the ICD due to changes in the coding rules. Clearly, this poses particular challenges for analysis of change over time and also between countries when there is a time interval in the adoption of a new version of the international classification.

However well a coding scheme captures the actual disease experienced by a population, there remains a serious difficulty in equating this with everyday experiences of health. There has been growing interest in overall measures of population health status that are globally comparable, although these require complex value judgements in making the link between specific levels of observed disease and overall well-being. Hausman (2012) presents a discussion of these issues, particularly examining the contribution of health to overall well-being.

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