Epidemiology

Epidemiology has been defined as "the study of the distribution and determinants of disease frequency in human populations" (Hennekens and Buring). Based on the underlying tenet that disease does not occur at random, this definition provides a framework for the systematic investigation of health, disability, and illness in human populations. Thus, epidemiology has been identified as the science that forms the basis upon which public health decisions can be made.

In its relatively short history, the field of epidemiology has evolved to reflect the changing nature of the primary causes of morbidity (illness) and mortality (death). The term "epidemiology" was first used to refer to the study of the great "epidemics" of the nineteenth century, such as cholera and diphtheria, in an era when infectious diseases accounted for the overwhelming majority of deaths. Improvements in nutrition, sanitation, housing conditions, and water quality in the early twentieth century resulted in the eradication of many infectious diseases and an increase in life expectancy. Today chronic diseases are emerging as the leading causes of death and disability in developed nations, and there is a shift in the demographic profile toward an increasingly aging population. In turn, the understanding of an epidemic has been broadened to reflect any disease that occurs at a greater frequency than would usually be expected in a specified population or geographic region. It has also been recognized that many chronic conditions have a consistent, endemic presence in a given population or geographic area, such as arthritis among older women. Other conditions are so widespread across a country, or even worldwide, that they are considered pandemic, such as obesity in the Western world.

Quantifying the occurrence of disease and its distribution in relation to the characteristics of person, place, and time falls within the domain of descriptive epidemiology. Measures of disease frequency rely on "count" information, the most basic of which is to add up individuals with a condition. However, for making direct comparisons among different groups it is necessary to determine the denominator, or the source population that the individuals come from. "Incidence" refers to the number of new cases of a condition that develop in a defined population within a specified time interval; it is a useful measure to study disease etiology. "Prevalence" refers to all occurrences of a condition in a defined population at a specific point in time; it is useful for health resource planning. Incidence and prevalence are interrelated in that prevalence is a function of both incidence and duration of a condition. For example, the prevalence of Alzheimer's disease exceeds the incidence of Alzheimer's disease because, once diagnosed, Alzheimer's patients live for many years with the disease. Prevalence is a "snapshot" that captures both newly diagnosed cases and previously diagnosed cases still alive. Institutionalized individuals are often excluded from the numerator (cases) and/or the denominator (persons at risk). Given that institutionalization rises dramatically with age, this poses a unique challenge for the reporting of morbidity rates among older age groups.