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Epidemiology it’s definition uses and steps

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EPIDEMIOLOGY - Definition, Uses, and Steps

(Source: Park's Textbook of Preventive and Social Medicine)

DEFINITION

The word "epidemiology" is derived from Greek: epi = among, demos = people, logos = study.

The most widely accepted definition (Last, IEA Dictionary) is:

"The study of the occurrence and distribution of health-related events, states, and processes in specified populations, including the study of the determinants influencing such processes, and the application of this knowledge to control relevant health problems."

Other notable definitions:

Frost (1927): "The science of the mass phenomena of infectious diseases."
Greenwood (1934): "The study of disease, any disease, as a mass phenomenon."
MacMahon (1960): "The study of the distribution and determinants of disease frequency in man."

Key words in the definition:

Study includes surveillance, observation, hypothesis testing, research, and experiments
Occurrence refers to disease frequency (incidence and prevalence)
Distribution refers to patterns by time, place, and person
Determinants refers to risk factors and causes
Application to control - makes explicit the ultimate aim of epidemiology: to promote, protect, and restore health

THREE COMPONENTS OF EPIDEMIOLOGY

1. Disease Frequency

Measurement of frequency of disease, disability, or death expressed as rates and ratios (prevalence rate, incidence rate, death rate). These rates are essential for comparing disease frequency across populations and subgroups, yielding clues to aetiology.

2. Distribution of Disease

Disease is not uniformly distributed in human populations. The epidemiologist studies distribution patterns by time, place, and person - examining:

Has disease increased or decreased over time?
Is there higher concentration in one geographic area?
Does it occur more in a particular age-group or sex?

This aspect is known as descriptive epidemiology and leads to generation of aetiological hypotheses.

3. Determinants of Disease

A unique feature is testing aetiological hypotheses and identifying underlying causes or risk factors. This is the real substance of epidemiology, known as analytical epidemiology. Analytical strategies help develop scientifically sound health programmes and policies.

AIMS OF EPIDEMIOLOGY (IEA)

According to the International Epidemiological Association, epidemiology has three main aims:

To describe the distribution and magnitude of health and disease problems in human populations
To identify aetiological factors (risk factors) in the pathogenesis of disease
To provide data essential to the planning, implementation, and evaluation of services for prevention, control, and treatment of disease

The ultimate aim is to:

Eliminate or reduce the health problem or its consequences
Promote the health and well-being of society as a whole

STEPS / USES OF EPIDEMIOLOGY

Morris (1957) identified seven distinct uses of epidemiology:

1. To Study Historically the Rise and Fall of Disease

Epidemiology provides a means to study disease profiles and time trends. Health and disease patterns are never constant. As old diseases (e.g., smallpox) are conquered, new ones (e.g., Legionnaires' disease, AIDS) emerge. By studying trends, we make useful projections and identify emerging health problems.

2. Community Diagnosis

Epidemiology serves as the "diagnostic tool" of community medicine. It identifies and quantifies health problems in terms of mortality and morbidity rates, identifies groups at risk, lays down priorities in disease control, and provides benchmarks for evaluating health services.

3. Planning and Evaluation

Epidemiological data provides the fundamental basis for:

Planning health facilities (hospital beds, health manpower), preventive services (screening programmes, immunization campaigns), and research
Evaluating control methods (e.g., cost-effectiveness of vaccines, evaluation of treatment modalities through randomized controlled trials)

4. Evaluation of Individual's Risks and Chances

Epidemiologists calculate relative risk and attributable risk for factors believed to cause disease. Examples include risk of bearing a mongoloid child, risk assessment for smokers vs. non-smokers for cancer and coronary heart disease.

5. Syndrome Identification

Epidemiological investigations define and refine disease syndromes by studying groups. Example: differentiation of gastric and duodenal ulcers based on the "poverty gradient" in gastric ulcer certification.

6. Completing the Natural History of Disease

The epidemiologist, by studying disease patterns in the community in relation to agent, host, and environment, fills gaps in the natural history of disease. Example: Epidemiology revealed that one-third to two-thirds of deaths from ischaemic heart disease are sudden (occurring within 1 hour) - a finding impossible from hospital studies alone.

7. Searching for Causes and Risk Factors

Epidemiology identifies causes by relating disease to inter-population differences. Notable contributions include:

Rubella as a cause of congenital defects
Thalidomide as a teratogenic agent
Cigarette smoking as a cause of lung cancer
Oxygen exposure in premature babies as a cause of retrolental fibroplasia

The concept of "risk factors" gave renewed impetus to epidemiological research, especially in chronic diseases.

EPIDEMIOLOGY vs. CLINICAL MEDICINE

Feature	Clinical Medicine	Epidemiology
Unit of study	Individual case	Defined population
Concern	Disease in the individual	Disease patterns in the population
Perspective	Diagnosis, prognosis, treatment	Distribution, causation, prevention
Data form	Clinical & lab findings	Tables, graphs, rates

Summary: Epidemiology is the cornerstone of community medicine and public health. Its definition encompasses measurement of disease frequency, description of distribution, and identification of determinants - all directed toward the single goal of disease prevention and health promotion in populations.

(Park's Textbook of Preventive and Social Medicine, pp. 64-104)

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