Epidemiology and Some Epidemiological Concepts

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“Epidemiology is the study of the distribution and determinants of disease frequency in man”(MacMahon and Pugh, 1970, p.1). It addresses such questions as: How many people now have this disease? More or less than last month, last year? What kind of people fall ill? Are they older or younger? Richer or poorer? More apt to be men or women? More likely to be living in one fashion than another? and so on. Epidemiology has had a long and important history in public health medicine, application to the field of mental health is more recent. Epidemiology is not, as the term suggests, limited to the study of epidemics or of communicable diseases but rather is concerned with any disorder. By studying the conditions which affect the distribution of a disease in a population, epidemiologists attempt to uncover the characteristics and causes of the disease and the effectiveness of attempted treatments. An early and dramatic illustration of the value of the epidemiological approach occurred during a cholera epidemic in London early in the last century. John Snow discovered that the victims lived close to a particular well from which they drew their drinking water. It seemed that something related to the well was causing the disease, although at the time nothing was known about the bacterial agent involved. When the well handle was removed and people could no longer drink the water, the number of cholera cases went down radically. In more recent times, epidemiological research has illuminated pellagra and lung cancer, in the one case discovering its relation to dietary deficiences in the southern United States and in the other the linkage to cigarette smoking. The epidemiology study of mental disorders, it is hoped, can similarly facilitate the work of the mental health field.

Some Epidemiological Concepts

Epidemiology has developed statistical methods to describe the rates, duration, and severity of disease in populations and their relation to conditions of the physical, biological, and social environment. The incidence rate is defined by the number of persons contracting the disease in a particular period of time. The prevalence rate describes the number of people who have the disease at any particular time. The difference between these rates is a function of the duration of the disease. For very short-acting conditions (say, the common cold) incidence and prevalence are quite similar; with longer-lasting conditions (say, schizophrenia) prevalence rates will clearly exceed incidence rates.

The distinction is important because the circumstances determining each of these rates may be different, although t’ae two taken together describe the overall health of the population. If two populations, distinguishable in some other regard, differ in one or another of these statistics, the explanation can be sought in terms of the ways they differ. Traditionally, public health workers have examined differences in the host, an agent of disease, or the environment. Host characteristics might include age, sex, physical condition (e.g., overweight), behaviors (e.g., smoking), race or social class, which might dispose persons toward a disease. Agent, in medical epidemiology, usually refers to pathogenic bacteria or viruses. Finally
, the environment in which the population lives may contain factors conducive to the development or continuation of diseases, in terms of factors as diverse as climate or
altitude, diet, or cultural norms. Study of environmental factors may bring to light ecological determinants which might have eluded clinical notice. This was illustrated by a study in London during a period of particularly intense fog in December of 1952 which showed that the death rate rose sharply along with the increasing concentration of air pollutants. Individual physicians working with their patients could not readily see the damaging effects of air pollution, for people did not die of “bad air” but rather of heart conditions and other medical diseases. Only the accumulation of epidemiological data revealed the nature and extent of the problem. Similarly, epidemiological study of the incidence of ulcer attacks among accountants in the United States shows a distinct peaking at the time of year that income tax retums must be filed, which illustrates the effects of social stress on that condition. Note, in these illustrations, that epidemiological research is best able to reveal empirical associations and to suggest hypotheses that then need to be pursued in clinical and experimental studies before the mechanism of the disease is known. To know what in the water supply causes cholera required years of further research; that something in the water caused the disease and that preventive action could be taken even without ultimate knowledge of the disease mechanism was demonstrated in Snow’s pioneering work.

Epidemiologists sometimes distinguish between descriptive and analytic epidemiology. The former consists of large-scale surveys describing incidence and prevalence of a disease along with numerous patient and environmental characteristics. Such studies can generate hypotheses as to the nature of the disease and suggest potential preventive and therapeutic programs. From such data, analytic epidemiology involves more controlled investigation of specific populations, agents, or environments to test the hypotheses developed earlier. Whether descriptive or analytic, however, epidemiological research usually has to be supplemented by research in other modes.

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