Epidemiology

One of the most frequently used definitions of epidemiology was given by MacMahon and Pugh (1970):
    Epidemiology is the study of the distribution and determinants of disease frequency in man.

The three components of this definition, i.e. frequency, distribution, and determinants embrace the basic principles and approaches in epidemiological research. The measurement of disease frequency relates to the quantification of disease occurrence in human populations. Such data are needed for further investigations of patterns of disease in subgroups of the population. This involves “…describing the distribution of health status in terms of age, sex, race, geography, etc., …”(MacMahon and Pugh 1970). The methods used to describe the distribution of diseases may be considered as a prerequisite to identify the determinants of human health and disease. This definition is based on two fundamental assumptions: First, the occurrence of diseases in populations is not a purely random process, and second, it is determined by causal and preventive factors (Hennekens and Buring 1987).

A more comprehensive definition of epidemiology as given by Last (2001):
    The study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to control of health problems.

In this broader sense, health-related states or events include “diseases, causes of death, behaviors such as use of tobacco, reactions to preventive regimens, and provision and use of health services” (Last 2001). According to this definition, the final aim of epidemiology is to promote, protect, and restore health. Hence, the major goals of epidemiology may be defined from two overlapping perspectives.
The first is a biomedical perspective looking primarily at the etiology of diseases and the disease process itself. This includes
FIRST, the description of the disease spectrum, the syndromes of the disease and the disease entities to learn about the various outcomes that may be caused by particular pathogens,
SECOND, the description of the natural history, i.e. the course of the disease to improve the diagnostic accuracy which is a major issue in clinical epidemiology,
THIRD, the investigation of physiological or genetic variables in relation to influencing factors and disease outcomes to decide whether they are potential risk factors, disease markers or indicators of early stages of disease,
FOURTH, the identification of factors that are responsible for the increase or decrease of disease risks in order to obtain the knowledge necessary for primary prevention,
FIFTH, the prediction of disease trends to facilitate the adaptation of the health services to future needs and to identify research priorities,
SIXTH, the clarification of disease transmission to control the spread of contagious diseases e.g. by targeted vaccination programs.

A similar description of the specific objectives of epidemiology as given by Leon Gordis (Epidemiology, 4th Edition,2009)
First, to identify the etiology or cause of a disease and the relevant risk factors—that is, factors that increase a person's risk for a disease. We want to know how the disease is transmitted from one person to another or from a nonhuman reservoir to a human population. Our ultimate aim is to intervene to reduce morbidity and mortality from the disease. We want to develop a rational basis for prevention programs. If we can identify the etiologic or causal factors for disease and reduce or eliminate exposure to those factors, we can develop a basis for prevention programs.
Second, to determine the extent of disease found in the community. What is the burden of disease in the community? This question is critical for planning health services and facilities, and for training future health care providers.
Third, to study the natural history and prognosis of disease. Clearly, certain diseases are more severe than others; some may be rapidly lethal while others may have longer durations of survival. Still others are not fatal. We want to define the baseline natural history of a disease in quantitative terms so that as we develop new modes of intervention, either through treatments or through new ways of preventing complications, we can compare the results of using such new modalities with the baseline data in order to determine whether our new approaches have truly been effective.
Fourth, to evaluate both existing and newly developed preventive and therapeutic measures and modes of health care delivery. For example, does screening men for prostate cancer using the prostate-specific antigen (PSA) test improve survival in people found to have prostate cancer? Has the growth of managed care and other new systems of health care delivery and health care insurance had an impact on the health outcomes of their patients and on their quality of life?
Fifth, to provide the foundation for developing public policy relating to environmental problems, genetic issues, and other considerations regarding disease prevention and health promotion. For example, is the electromagnetic radiation that is emitted by electric blankets, heating pads, and other household appliances a hazard to human health? Are high levels of atmospheric ozone or particulate matter a cause of adverse acute or chronic health effects in human populations? Is radon in homes a significant risk to human beings? Which occupations are associated with increased risks of disease in workers, and what types of regulation are required?

流行病学是研究人群中疾病与健康状况的分布及其影响因素，并研究防制疾病及促进健康的策略和措施的科学

Since the definition had been put forward in 1992(Lian ZhiHao, Epidemiology 3rd, 1992,Beijing),the definition expresses the similar meaning of the definition from Last(2001), so it has been used up to now in China. The current definition of epideology not only well reflect the practice of public health in China,but also fully demonstrates the nature of the descipline, so the definition has been used in the text book in China for many years.