I. The Epidemiology of Chronic Bronchitis

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Abstract
The science of epidemiology has come to mean the study of disease as a mass phenomenon. Its purpose is to disclose the causative factors of disease and thus to lead to prevention. It has achieved its major successes in the past in the field of contagious illness in which it has fulfilled its literal meaning of a study of something which falls upon people. Now, however, its disciples are turning their attention to non-contagious illness and to chronic conditions which develop rather than fall upon us. It is generally recognised that chronic disease processes are often due to multiple rather than to single causes and the task of the epidemiologist in chronic disease is to unravel the intricacies of the various aetiological factors which may operate singly or in combination. The epidemiologist's unit of study is an aggregate of human beings—some fraction of the population which has been deliberately segregated by the observer. The unit may be assembled quite at random as by compiling a list of names from an electoral register, or it may be all persons of the same age or sex, or those employed in the same occupation, or anyone of a thousand different characters. One then studies certain features of medical interest in the unit by defining their prevalence or frequency of occurrence and compares the findings with those in other units or the whole population excluding the unit.

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