1. Understanding epidemiologic concepts
Understanding epidemiologic concepts
Epidemiology is the underlying and basic science of Public Health. A number of definitions have been used, but the following, adapted from the Center of Disease Control (CDC) “Principles of epidemiology, 3 rd Ed, updated 2012  , captures the underlying principles and public health connotation of epidemiology:
“Epidemiology is the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems.”.
An abbreviated description of the terms used in this definition is given below:
Distribution: Epidemiology is concerned with the frequency and pattern of health events in a population
Frequency refers not only to the number of health events such as the number of cases of diabetes in a population, but also to the relationship of that number to the size of the population. The resulting rate allows epidemiologists to compare disease occurrence across different populations.
Pattern: refers to the occurrence of health-related events by time, place, and person. Time patterns may be annual, seasonal, weekly, daily, or any other breakdown of time that may influence disease occurrence. Place patterns include geographic variation, urban/rural differences, etc. Personal characteristics include demographic factors which may be related to risk of illness, such as age, sex, and socioeconomic status, as well as behaviours and environmental exposures. Characterizing health events by time, place, and person are activities of descriptive epidemiology.
Determinants: any factor, whether event, characteristic, or other definable entity, that changes a health condition or other defined factors. Epidemiology also searches for factors, which are the causes for or that influence the occurrence of disease and other health-related events. The assumption is that illness does not occur randomly in a population, but happens only when the right accumulation of risk factors or determinants exists in an individual. Epidemiologists use analytic epidemiology or epidemiologic studies to provide the “Why” and “How” of such events. They assess whether groups with different rates of disease differ in their demographic characteristics, genetic or immunologic make-up, behaviours, environmental exposures, or other so-called potential risk factors.
Health-related states or events: Epidemiology was originally focused exclusively on epidemics of communicable diseases  but throughout time continued to expand, among others addressing non-communicable infectious diseases, chronic diseases, injuries, birth defects, maternal-child health, occupational health, environmental health, behaviours related to health, and recently examining genetic markers of disease risk. Indeed, the term health-related states or events may be seen as anything that affects the well-being of a population. Nonetheless, many epidemiologists still use the term “disease” as shorthand for the wide range of health-related states and events that are studied.
Specified populations: Epidemiologists are concerned about the collective health of the people in a population. In other words, the clinician’s “patient” is the individual; the epidemiologist’s “patient” is the community. Specified populations are specific parts of the overall community, which share predefined characteristics that are of interest for a certain study. This may be a group of persons who have been exposed to a specific risk or a group of persons of a certain age. The epidemiologist focuses on identifying the exposure or source that caused the illness; the number of other persons who may have been similarly exposed; the potential for further spread in the community; and interventions to prevent additional cases or recurrences.
Application: Epidemiology is not just “the study of” health in a population; it also involves applying the knowledge gained to community-based practice. The epidemiologist uses the scientific methods of descriptive and analytic epidemiology as well as experience, epidemiologic judgment, and understanding of local conditions in “diagnosing” the health of a community and proposing appropriate, practical, and acceptable public health interventions to control and prevent disease in the community. Thus, application refers to using the gained knowledge to provide guidance for public health questions.
Typical questions an epidemiologist might ask:
- How many are affected by such events?
- What is the relevance of the problem?
- Is the risk increasing or decreasing? How could it be prevented?
Broadly speaking, the activities epidemiologists are engaged in and apply include:
- Surveillance and descriptive studies that can be used to study and describe the health status, disease distribution, disease occurrence or other characteristics of specific populations. The results are applicable to the specific target populations that are studied.
- Inquiry into causes of illness. Results apply not only to specific target populations but are generalised to cover types of people that haven’t yet been studied but for whom the assumption is that the results would apply.
- Helping with study design, collection, and statistical analysis of data, and interpretation and dissemination of results.
- Developing methodology used in clinical research , public health studies, and, to a lesser extent, basic research in the biological sciences.
- Identification of risk factors for diseases and targets for preventive healthcare.
- Shaping policy decisions and evidence -based practice and the support of the implementation of public health measures.
- Mathematical modelling. This can project how e.g. infectious diseases progress to show the likely outcome of an epidemic and help inform public health interventions. The modelling can help decide which intervention(s) to avoid and which to trial, or can predict future growth patterns, etc.
 Principles of epidemiology, 3rd Ed, updated 2012.Principles of epidemiology in public health practice; an introduction to applied epidemiology and biostatistics. 3rd ed (cdc.gov)
 Greenwood M.Epidemics and crowd-diseases: an introduction to the study of epidemiology, Oxford University Press; 1935.