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Epidemiology is a population’s health science. It utilizes a population approach for the study of the distribution (person, place, and time) and determinants (biological, social) of health and disease in defined populations of varying characteristics, and how to use the information for the prevention and control of various health problems.

The term epidemiology is of Greek origin and can be roughly translated as the study among or upon people. The origins of epidemiology can be traced back to the era of Hippocrates, when the idea that environmental factors (seasons, winds, hot, cold) can influence disease occurrence was in circulation. However, epidemiology’s development into a full-fledged discipline excelled in the 19th century with the work of John Snow, on which identified the relation between drinking water supplied from a certain company and the risk of death from cholera. Although epidemiology originated from the study of communicable diseases, it eventually developed a more comprehensive scientific approach to studying various health-related states including noncommunicable diseases, disability, accidents, quality of life, and others.

An epidemiologist is a scientist who uses epidemiological methodology to investigate various phenomena related to the health of the population. An epidemiologist should also have some knowledge of other public health disciplines, statistics, and social and medical sciences. An epidemiologist’s range of functions includes practical applications, such as outbreak investigation and field epidemiology, in addition to applications such as formulating and testing epidemiological hypothesis and developing study designs. However, in all efforts, the epidemiologist aims to use epidemiological thinking and methods to contribute to disease prevention and health promotion.

Traditionally, epidemiology has been classified by type of discipline or disease and physiology. Examples of epidemiology discipline classifications include: environmental, social, pharmacoepidemiology, nutritional, genetic, molecular, and clinical and surveillance. Examples of disease and physiology – based classification include: reproductive epidemiology, epidemiology of aging, cancer epidemiology, and injury epidemiology.

Epidemiology attempts to answer various questions regarding the distribution of diseases and the determinants of health, such as: How many people developed the disease? What is the disease burden in a certain population? Why a specific group of the population developed the disease while the others did not? What are the factors associated with disease? What are the different stages of disease? What is the prognosis? Is there a causal relation between a certain factor (exposure) and the disease? Are the interventions used to prevent or control the disease effective? What are the public policies that should be formulated and the regulations to be applied to safeguard the health of the population?

Answering the Questions

In its attempt to provide scientifically sound answers to theses questions, epidemiology adopts two main approaches. Descriptive epidemiology focuses on studying the occurrence of disease, disability or any other health-related phenomenon. It observes and describes the relation of the disease with the basic population characteristics such as age and sex. The person, place, and time triad is the cornerstone of descriptive epidemiology. It does not aim to tests hypotheses, for example to prove or disprove a causal relation. In contrast, analytical epidemiology usually studies causal relations, tests hypotheses, and measures the association between exposures and outcomes.

Measuring and comparing the occurrence of diseases and death is achieved by using various measurements of morbidity and mortality. Prevalence refers the total number of persons with the disease or health related event during a defined period or point in time, and the prevalence rate is calculated by dividing this total number of cases or persons with the disease by the population at risk of having this event. Incidence refers to the new events or cases in a defined period only and the incidence rate is calculated by dividing the new cases by the population at risk at the same period, and can be expressed as person-time. Comparing the occurrence of disease among groups of people with different exposure status (exposed vs. unexposed) is useful to show the effect and to calculate the risk of being exposed to a certain factor (exposure) on a health outcome (disease). Both absolute (risk difference) and relative (risk ratio) comparisons of risk are available. Various measures can be used to assess mortality such as the crude mortality rate, which is calculated by dividing the number of deaths in a defined period by the average total population in the same period. Other measures of mortality include age-specific and age-adjusted mortality rates, which allow comparison of rates among populations with different age structures.

Epidemiological Study Designs

Answering these different epidemiological questions requires the use of suitable epidemiological study designs, which are classified into observational and experimental studies. Experimental studies involve an intervention introduced or eliminated by the researcher/epidemiologist, while the observational studies are based on observation and measurement only. Examples of observational studies include ecological, cross-sectional, case-control, and cohort studies. A cross-sectional study can be used, for example, to determine the prevalence of diabetes mellitus in a certain population. Experimental studies include the randomized controlled trial, which is considered the gold standard of epidemiological designs, field trials, and community trials. An example of experimental study application is the use of a randomized controlled trial to study the effect of a new oral hypoglycemic drug on controlling blood glucose in persons with diabetes mellitus. Epidemiological studies vary in their characteristics such as the capacity to study causal relations, the cost of conducting a study, the duration, the required sample size, and other characteristics. These characteristics affect the choice of a certain epidemiological study over the other.

Epidemiological study errors that can affect the accuracy of collected information include random error, systematic error (bias) and confounding. Minimizing the effects of those errors is an important component of epidemiological study design and analysis.

Surveillance

Another important aspect of epidemiology is surveillance, which involves a continuous systematic process that involves data collection, analysis, interpretation and dissemination of results. Data collected include information on diseases, risk factors, complications, and health practices. Surveillance objectives include detecting epidemics, monitoring disease trends, identifying risk factors and the emergence of new diseases and microbes.

Outbreaks are usually marked by an unexpected rise in the incidence of a certain disease above the base-line level. Outbreak investigation classically originated from communicable diseases outbreaks such as typhoid. In the modern age, different types of outbreaks have been identified including outbreaks due to environmental factors such as contamination with chemicals. An example of an environmental pollution epidemic is the Minamata Bay-Japan case, where methylmercury accumulated in fish due to the release of chemicals containing mercury from a nearby factory. This caused severe poisoning among fishermen and their families who consumed fish as a main food item. Epidemiology played a crucial role in identifying the cause of and in controlling such epidemics.

Bibliography:

John Last, A Dictionary of Epidemiology (Oxford University Press, 2000);
Coggon, Geoffrey Rose, D. Barker, Epidemiology for the Uninitiated (BMJ Publishing Group, 2003);
Beaglehole, R. Bonita, T Kjellstrom, Basic Epidemiology (World Health Organization, 1993);
Leon Gordis, Epidemiology (W.B. Saunders Company, 2004);
Raj Bhopal, Concepts of Epidemiology (Oxford University Press, 2002).