Disease, Concepts and Classification of

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The classification of diseases is the subject of the branch of medicine called nosology. Contemporary classification efforts range from those primarily intended to clarify and standardize the nomenclature of diseases, such as the College of American Pathologists' Systematized Nomenclature of Medicine (SNOWMED) and the National Institute of Health's Unified Medical Language System (UMLS), to taxonomies focused on diagnostic terminology that emphasize an ordered hierarchical system reflecting natural relationships between diseases, such as the World Health Organization's International Statistical Classification of Diseases and Related Health Problems (ICD).

The Early History of Disease Classification

The classificatory approach to disease is ancient, but the classification of diseases in a modern sense dates roughly from the fifteenth and sixteenth centuries. The concept that diseases are directly identifiable through their symptomatology is most evident in the revolutionary work of Paracelsus (born Theophrastus von Hohenheim, 1493–1541). Paracelsus's suggestion that natural symptoms may provide direct evidence leading to a probable diagnosis or classification of a disease despite a potentially unobservable cause of the illness was evident in practice by the seventeenth century, when nosology took form.

Throughout the eighteenth century the pathological view was guided by the concept that a specific cause could be identified for all episodes of ill health, and physicians concentrated on the observation of symptoms and the categorization of disease. Even the inevitability of death from natural causes and the presumption of a biologically acceptable risk of infant death gave way to perceptions that a cause for all illnesses could be diagnosed and that these causes of disease constituted social problems that could be resolved.

Swedish naturalist Carolus Linnaeus (1707–1778), best known for his botanical classifications, attempted to provide a systematic classification of diseases during that period. His classification is no-table for including a broad range of morbidity conditions, including functional health limitations (loss of movement, impeded motion, etc.) and mental health conditions as they were understood at the time (delirium, melancholia, bulimia, etc.). In a fashion similar to his botanical classification, Linnaeus designated genera, species, and subspecies of disease. Not surprisingly, his classification is not predicated on a single classificatory principle. Linnaeus did emphasize symptomatology. However, his classification reflects elements of alchemy, astrology, and the prevailing miasmatic environmental view of disease. Other, similar classification attempts by individual physicians of that time (e.g., Francois Bossier de Lacroix's Nosologia Methodica and William Cullen's Classification) were not theoretically grounded and did not embody a clear notion of symptoms derived from causal agents. Arguably, the effect was to produce complex nosologies, competing classifications, and confusion.

The Nineteenth Century and the Advent of Germ Theory

Competing conceptions of disease were and still are also rooted in strongly held social hypotheses and are debated in the context of political, economic, and religious interests. The establishment over the course of the early nineteenth century of various "centres of calculation" in England, including the General Register Office (GRO), the Alkali Inspectorate, and the inspectors of the Passenger Acts and Factory Acts, provided a governmental context for often lively debates about the development of morbidity and mortality measurement (Bartley et al.1997). Social Darwinists, using the concepts developed by the scientist Francis Galton, even challenged the wisdom of efforts directed at reporting and eradicating all diseases, especially the apparently "selective" preponderance of infant deaths among the poor. The rising influence of insurance companies and the actuarial trades generated corporate and economic interest in accurate statistics. However, related implications for the payment of benefits probably added to local and familial pressures to record and report information selectively. Religious institutions, including both the Anglican Church and non-Anglican denominations, had a vested interest in the registration of vital events and in conceptions of disease that included moralistic and attributional causation. Even in the present time diseases are often considered to be caused by moral failings on the part of those who suffer their effects.

The early foundations of disease classification reflected both the sociopolitical context of the nineteenth century and the contest between highly moralistic conceptions of the underlying cause of disease and principles of scientific reporting and classification. Popular conceptions of disease in turn influenced many public health initiatives, which frequently were directed toward improving the moral behaviors (idleness, drunkenness, etc.) of the lower classes that were considered the cause of their suffering.

Despite an early understanding of the infectious nature of many diseases, through most of the nineteenth century classifications of disease were not informed by the concepts of germ theory. Instead, the major prevailing concepts of disease were grounded in the remnants of a miasmatic view of disease. For many diseases the idea of disease vectors and infectious transmission came slowly. Poisonous vapors, atmospheres, environments, and toxins were conceived to be responsible for even infectious diseases. Edwin Chadwick's etiological hypothesis, which stated that poverty and environmental conditions including sanitation were primarily responsible for harmful disease environments, refined the miasmatic view and shaped the sanitary movement in England.

In midcentury, the prevailing nosological classification treatise in America was Daniel Drake's Systematic Treatise, which arranged diseases into five large classes (autumnal fevers, yellow fever, typhus fevers, eruptive fevers, and phlogistic fevers) and was strongly influenced by a miasmatic or environmental conception of disease. Drake's Treatise differed, however, from the work of many of his predecessors in visualizing at least some diseases (e.g., cholera and malaria) as being due to minute animalcules or germs that typified various environments. In fact, germ theory represented a rapid but not unanticipated transformation. Many physicians and scientists were rethinking the pathology of diseases over the last half of the nineteenth century. The two names that are most commonly associated with the new germ theory are those of Robert Koch and Louis Pasteur. Koch's bacterial swab test for the tuberculin bacillus and Pasteur's work on surgical septicemia and rabies were among the major medical developments that quickly altered both clinical practice and the widespread conceptions of disease among practitioners. The bacterial age of medicine, perhaps more than any subsequent medical development, altered fundamental conceptions of disease. The external miasmatic locus of disease became internal, the moral became the biological, the natural became seemingly preventable, and a new medical science was enshrined in both concepts and regulatory institutions.

Regulation of Statistical Reporting and the Evolving Quality of Classifications

The later nineteenth century was also a period of dramatic change in the statistical reporting of diseases. After the British adoption of civil registration (1837) and the formation of the American Statistical Association in Boston (1839), Massachusetts in 1842 became the first state to implement a modern record-keeping system for deaths and cause of death information in the United States. Over the next half century changes in both reporting institutions and classifications of disease were dramatic. For example, the nomenclature for tuberculosis, the leading nineteenth-century killer throughout much of the Western world, evolved in regard to recorded deaths, as shown in Table 1 for two Massachusetts mill towns, Northampton and Holyoke. In 1850 all these deaths were recorded as being due to consumption. By 1880 the vast majority were labeled phthisis, a term that had long been in use. Eventually, after the advent of Koch's bacterial test and an international classification of diseases (1900), virtually all such deaths were labeled using the still-current nomenclature of pulmonary tuberculosis.

Despite the evolving nomenclature, the recorded causes of death also demonstrate continuing social biases. Gender, ethnicity, occupation, and wealth and other social distinctions were associated with both the likelihood of attributing specific diagnoses to individuals and the quality of reported diagnoses as modern classification systems evolved. Three-quarters of the reports of deaths from unintentional injuries to men, for example, gave informational circumstances of the accident, while only a third of those for women included such qualifying information. Even for infectious diseases within similar age ranges, such as tuberculosis deaths in persons age 15 to 64, men were significantly more likely to have more detailed diagnoses recorded. Riess (1997) identifies the major changes in recording the causes of death over the nineteenth century as being due to (1) the shifting ecology of disease, (2) shifting definitions of health and disease, (3) shifting definitions of death and causes of death, (4) the changing construction of death records, and (5) changes in the use of medical statistics. To these factors should be added the dramatic changes in population composition such as those resulting from the aging, urbanization, immigration, and industrialization of the Western world.

The ICD Family of Disease Classifications and Professional Nosology

As medical science and reporting systems advanced, so did demands for a standardized classification of diseases for both clinical and statistical purposes. A number of medical statisticians, including William Farr (1807–1883), the Register General of the GRO, and French physician Jacques Bertillon (1851–1922), attempted to provide more refined classifications of disease and establish the principles on which classifications were based. The notable efforts of Bertillon to refine a classification of diseases using principles informed by germ theory were dramatically advanced when the French government convened in 1900 the first international conference to revise and promote the Bertillon International Classification of Causes of Death (ICD). Successive conferences were held by the French government in 1909, 1920, 1929, and 1938. The continued standardization and development of a central family of classifications was ensured in 1946 when the International Health Conference entrusted the World Health Organization (WHO) with the responsibility for the sixth revision of the International Lists of Diseases and Causes of


Death and expanded the classification to include nonfatal diseases.

The WHO has maintained responsibility for the ICD family of disease classifications, the most widely used system for nosology. The ICD (current version in use in 2003 is 10) is a generic disease classification used for most cause of death nosology (list of diseases). However, clinical modifications (currently ICD 9cm) and special-purpose nosologies (e.g., ICD-0 for oncology and ICF for functioning, disability, and health), modeled after the ICD, are also widely used. As demands for contextual and qualifying information have increased in the last several decades, the basic ICD codes for diseases have been extended with additional qualifying codes. For example, V codes were added to record reasons for encounter or factors related to health status, E codes to record external causes of injury and ill health, M codes for the morphology of neoplasms, and so on. This entire ICD family of classifications is maintained by the WHO in cooperation with collaborating centers throughout the world that sometimes offer their own embellishments.

Although disease classification systems have a long history, professional nosology involving the coding of diseases from literal diagnoses expanded dramatically in the last half of the twentieth century. Many hospitals began experimenting with clinical use of the 1948 ICD. In 1962 the U.S. Public Health Service produced an adaptation (ICDA) for use in hospital records. This was followed by ICD8 and ICD9cm for clinical use. However, nosologists were often considered clerical workers who performed a necessary but only tolerated task. The profession achieved stature in the late twentieth century as the critical importance of the efforts of its practitioners to medical research became clear. However, nosology has begun to decline once again in the face of an increasing use of electronic records and the growing promise of automated nosology.

The Predictability of Change in Concepts and Classifications

The evolving history of disease and the growth in knowledge of diseases guarantee continual change in both concepts and classifications of diseases. Those conceptions and classifications have experienced and reflected radical changes in underlying etiologies, shifting sociopolitical environments, the changing composition of host populations, and the growing social organization of medicine. Just as the advent of germ theory had a strong influence on conceptions and classifications of diseases in the nineteenth century, advancing knowledge of diseases will continue to change conceptions and classifications in the twenty-first century.

The genomic revolution, bringing profound advances in understanding the genetic foundations of disease, is one likely source of such change (Cantor and Smith 1999). Another source is the still-controversial advances in evolutionary biology, which threaten to erode entrenched boundaries between infectious and noninfectious, or chronic and acute, diseases (e.g., Ewald 2000). A third source of change will be trends in the incidence and nature of diseases themselves and in their host populations. The growing impact of poorly defined chronic conditions that are relegated to residual, symptomatic diagnoses, such as Alzheimer's disease, fibromyalgia, Gulf War syndrome, and chronic fatigue, challenges existing concepts and classifications. The increasing age, growing urbanism, declining fertility, and changing socioeconomic and occupational composition of the world's population will alter the emphases placed on different groups of diagnoses. Even among conditions that once were assumed to be well understood, such as childhood asthma, trends in disease challenge existing etiologies. Indeed, an overly rigid adherence to diagnostic classifications inherited from the nineteenth century may occasionally be an obstacle to future growth in the understanding of disease, just as heavily moral and religious conceptions of disease retarded the advancement of germ theory and the development of current concepts and classifications.

See also: Bertillon, Jacques; Causes of Death; Disease, Burden of; Farr, William.


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Douglas L. Anderton