What epidemiology is
- Concept
Epidemiology studies how much disease a population has, who gets it, where and when it appears, and why.
What it is
Epidemiology is the study of the distribution and determinants of health-related states and events in specified populations, and the use of that knowledge to control health problems. Descriptive epidemiology answers who, where and when (person, place and time) and generates hypotheses. Analytic epidemiology compares groups to answer why and how, and tests those hypotheses.
Structure
- Person: age, sex, occupation, ethnicity, socioeconomic level, immune status and behaviours.
- Place: country, department, municipality, neighbourhood, urban or rural area, workplace or school.
- Time: long-term trends, seasonality, clusters over days or weeks, and the epidemic curve.
Worked example
- Data (made up): the town of San Remigio has 20 000 inhabitants and recorded 60 dengue cases in 2025.
- Overall frequency: 60 / 20 000 = 0.003, that is, 3 cases per 1 000 inhabitants in the year.
- Person: 36 of the 60 cases are aged 5 to 14: 36 / 60 = 60 % of cases.
- Place: Barrio Norte has 40 cases in 5 000 residents (8 per 1 000); the rest, 20 in 15 000 (≈ 1.3 per 1 000).
- Time: 45 of the 60 cases (75 %) began between March and May, in the rainy season.
- Descriptive hypothesis: many homes in Barrio Norte store water in uncovered tanks, where mosquitoes breed.
- Analytic, in Barrio Norte: homes with an uncovered tank, 30 cases in 2 000 people (15 per 1 000); covered, 10 in 3 000 (≈ 3.3 per 1 000).
- Relative risk: 0.015 / 0.00333 = 4.5.
- Reading: living with an uncovered tank is linked to a 4.5-fold risk; description suggested the cause and comparison measured it.
Descriptive versus analytic epidemiology
| Feature | Descriptive | Analytic |
|---|---|---|
| Question | Who, where, when? | Why, how? |
| Comparison group | Not needed | Essential |
| Output | Hypotheses, service planning | Measures of association (RR, OR) and causal evidence |
| Examples | Case report, case series, ecological study, surveillance analysis | Cohort, case-control, clinical trial |
Markers
- Person
- Place
- Time
- Descriptive
- Analytic
In practice
- Surveillance data (such as the reports that reach SIVIGILA in Colombia) are descriptive epidemiology: they show where and when to act.
- Every clinical practice guideline rests on analytic epidemiological studies: reading them is part of medical practice.
Common pitfalls
- Comparing raw case counts between places of different size: rates (cases per population) are needed.
- Believing a descriptive association already proves cause: a comparison is still needed, and bias and confounding must be ruled out.
Test yourself
Think of the answer, then tap the question to see it.
What are the two main objects of study of epidemiology in its classic definition?
The distribution (frequency and pattern by person, place and time) and the determinants (causes and risk factors) of health events in populations.
What sets an analytic study apart from a descriptive one?
The analytic study has a comparison group and tests a hypothesis; the descriptive study describes by person, place and time and generates hypotheses.
Why compare rates rather than raw case counts between two neighbourhoods?
Because a more populated neighbourhood has more cases even with the same risk; the rate divides cases by the population at risk.
Connections
Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.
Sources: Principles of Epidemiology | Lesson 1 - Section 1 (Centers for Disease Control and Prevention, 2012).
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Representative model, for educational purposes only.