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CONCEPTS OF DISEASE AND DIAGNOSIS* DANA D. COPELANDf Medicine is the science and art of preventing, alleviating, or curing disease. Through diagnosis the physician identifies and names disease. While the concepts of disease and diagnosis are pivotal for medical science , philosophical ambiguities in both concepts persist and have great import for contemporary nosology. Unified concepts of disease have been built either on emphasis of exogenous causes of disease —xenochthonous models, or on emphasis of endogenous causes of diseases—autochthonous models. The choice between the two models is more than academic, since the derivation of diagnostic algorithms from the two models can be shown to differ. Xenochthonous Model of Disease At the time of Hippocrates, disease was treated as a purely clinical phenomenon and identified by descriptions ofobservable changes in the patient, for example, "fever," "cyanosis," and "consumption" [I]. Not until the seventeenth century and the introduction of an ontologie theory of disease by Thomas Sydenham did the notion of diseases as specific entities, and not merely the irregular manifestations of a disordered and debilitated nature, gain acceptance [2]. In his contention of disease specificity, Sydenham was strongly influenced by the emergence of the science of taxonomy; his debt to the Linnaeans is clearly illustrated in the preface to the third edition of his Medical Observations concerning the History and Cure of Acute Diseases: "In the first place, it is necessary that all diseases be reduced to definite and certain species, and that, with the same care which we see exhibited by botanists in their phytologies; since it happens, at present, that many diseases, although *This essay was submitted in the firstPerspectives Writing Award competition for authors under 35. tDepartment of Pathology, Duke University Medical Center, Durham, North Carolina 27710. 1 thank Linda Brogan for the typing ofthe manuscript; Dr. Robert Farnham, Dr. F. Stephen Vogel, and Dr. R. B. Jennings for review and valuable suggestions; Dr. W. P. Vaughan and Dr. David Ross for discussions which helped formalize the hypotheses; Dr. John A. Moore for authorizing and supporting the study; and Karen Copeland for her patience and encouragement during the writing of the essay. 528 I Dana D. Copeland ¦ Concepts ofDisease and Diagnosis included in the same genus, mentioned with a common nomenclature, and resembling one another in several symptoms, are, notwithstanding, different in their natures, and require a different medical treatment" [3, pp. 514-515]. Sydenham's eighteenth-century successors generated various nosological schema which have long since fallen from use, since their rules for classification were too frequently arbitrary and the assignment of a patient's malady to any given diagnostic category, while providing a self-contained explanation of illness, rarely provided any additional prognostic or therapeutic power. The formulation of a successful taxonomic strategy awaited the advent of the morbid anatomists, led by Bichat and Morgagni, armed with the techniques ofthe necropsy. The data of gross pathology permitted a demarcation of disease according to the morphological changes in the internal organs of the body. The histopathologists, and Virchow in particular , were further successful in introducing the concept that causality should be the foundation of nosology. The importance of cause, or etiology, was reinforced by the success of Pasteur's germ theory, which provided through the taxonomy of the microbiologist a totally exogenous identification ofmany disease etiologies. The nature ofthe etiological agent of pneumonia could, for example, be characterized in the test tube as well as in the lungs of the patient. The concept that disease represents the interaction between a healthy host and an extraneous cause (fig. 1) seemed at last verified, and a unified concept of disease appeared close at hand. The application of a homologous paradigm to any disease could be effected simply by the identification of the etiology and description of the pathogenesis. Scadding has succinctly summarized the three major premises of the paradigm as follows: "(a) Patients become ill because they are attacked by a disease or diseases, (b) Diseases are of sufficiently similar kind or nature to be classified in a CauseDisease Host Clinical Data Therapy*----------- Diagnosis -----------»Prognosis Fig. 1.—The basic xenochthonous model of disease and diagnosis Perspectives in Biology and Medicine ¦ Summer 1977 | 529 simple logical scheme, (c) The aim...

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