Epidemiology

= Epidemiology = toc

**Definition**
The study of the causes, distribution, and control of diseases at the population level. Epidemiology forms the core of local public health policy as well as the study of health disparities. Epidemiologists attempt to determine what factors are associated with diseases (risk factors), and what factors may protect people against diseases (protective factors). Epidemiologists must take a multi-dimensional approach to assessing a health problem in order to more accurately understand why certain diseases are prevalent in a specific population. There are numerous factors that affect the health of a population including biological factors (genetics), environmental factors (air we breathe, chemicals we are exposed to), cultural characteristics (amount of sleep we get, differing male and female roles), social and economic status (stress levels, quality of healthcare), and dietary patterns (nutrition). These factors are all inter-related. Hence, it is crucial for epidemiologists to use gathered data and integrate biomedicine, socio-cultural anthropology, and biological anthropology to identify factors that cause increased risk in certain populations.

Epidemiology is often viewed as a science based solely on statistical tools and genetic causes. As a result, genetic differences between populations are thought to be the primary reason for health disparities in the United States as well as across the globe. Citing genetic factors as primary reasons for varying health conditions of populations, however, is often incorrect. Most genetic variation in humans is found within, not among populations, meaning that genetic explanations for disease disparities among populations are overemphasized. Genetic differences between populations may be prevalent, but socio-cultural and psychosocial differences are crucial and must be considered in all cases.

History
Epidemiology evolved gradually as individuals attempted to explain in a systematic way how diseases occurred. Historic epidemics and pandemics such as the plague (Black Death), leprosy, and cholera, encouraged the direction of efforts to control human diseases and suffering. Epidemiology is based on the assumption that diseases go through several passes of development. This sequential categorization of disease progression is a concept known as the “Natural History of Disease”. Epidemiologists acknowledge that there are variations in the cycles of different diseases and in the manifestations in individuals. However; four common stages are accepted as the stages that most diseases will show: 1) Stage of susceptibility, 2) Stage of pre-symptomatic disease, 3) Stage of clinical disease, 4) Stage of diminished capacity. The field of epidemiology was gradually developed as theories of disease causation were developed, refined and tested.

Hippocrates is considered as the first epidemiologist, as he is recognized to be the first person that provided explanations for disease on a rational basis vs. supernatural explanations (5). Girolamo Fracastoro, John Graunt, Thomas Sydenham, James Lind, Edward Jenner, William Farr, John Snow, Ignas Semmelweis, Peter Ludwig Panum, Florence Nightingale, Louis Pasteur, Robert Koch, Anna Wessels Williams, and Joseph Goldberger are a few the most important and influential figures in the development of epidemiology as a discipline. John Graunt evaluated and described the Bills of Mortality in a systematical way for the first time. William Farr developed, expanded and applied methods of statistical analysis for epidemiology. John Snow applied epidemiological methods to discover the source and mode of transmission of cholera; and Joseph Goldberger demonstrated the cause of pellagra by means of observation and experimentation.
 * Key figures**

Types of Epidemiology
Describes the occurrence and distribution of disease and health related characteristics in human populations. These characteristics are generally categorized under “persons”, “place”, and “time”. Descriptive observations are necessary to develop hypotheses based on patterns and trends present within and among populations. Descriptive epidemiology is a fundamental part of disease surveillance. Descriptive studies include case reports, case series, ecological studies, and cross-sectional studies with no hypothesis testing.
 * Descriptive epidemiology**
 * Analytic epidemiology**

Examines and test hypotheses for causal relationships between observations and health outcomes. Etiological hypotheses are generated and tested; providing understanding for the development of mechanisms of disease causation. These analyses consist in the identification and measurement of determinants and the probabilities (risk factors) of disease development. Analytic studies are observational and not experimental. Some examples are some ecological and cross-sectional studies, case-control studies, prospective cohort studies, retrospective cohort studies, and hybrid studies. Also test hypotheses like the analytic studies. However; in a experimental study the investigator has direct control, manipulation or intervention on the conditions of the study. The investigator directly controls the status and level of exposure. The two main types of experimental studies are: randomized control trials (clinical trials) and community trials. These studies might have serious ethical implications (5).
 * Experimental studies **

Examples
For example, evaluating the health changes resulting from the social and ecological changes among Indigenous Siberian Populations requires studying the evolutionary past (adaptations to the cold, marginal environment that lead to elevation in basal metabolic rates), cultural characteristics (men and women in traditional populations do more similar amounts of physical work, but as lifestyle changes occur, men do most of the physical work), nutrition patterns (children experience malnutrition because of lower quality of foods), and physiological components (increased levels of adiposity, plasma lipid levels of native Siberians remain low, whereas blood pressure is elevated) of those people. 1 Hypertension is a disease frequently explained as a result of genetic makeup of certain populations. The slavery hypothesis is formed on the basis of genetic variation among groups, stating that the high frequencies of hypertension found among Afro-US populations are due to genetic reasons that evolved as a result of African slaves adapting to conditions of dehydration and low-sodium content. This hypothesis is tested against by comparing the high hypertension prevalence rates (HPR) of different populations of African origin in varying regions of the world. The results indicate that there are numerous African groups with low HPR; hence, genetic reasons cannot be the underlying reason for the high hypertension prevalent in African-Americans in North America. The slavery hypothesis does not take into consideration the wide range of socio-cultural and psychological differences among African groups. Possible social factors that affect HPR among African Americans are biological and psychological stressors caused by an urbanized environment, social inequality, poor nutrition, etc. The slavery hypothesis overemphasizes the role that genetic factors play while assessing health disparities between groups. In fact, population genetics studies show that approximately 85% of human genetic variation is found within, not between ethnic groups.

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