Outbreak investigation
- Concept
Investigating an outbreak means confirming it, counting and describing cases, finding the source, cutting it off and sharing what was learned.
What it is
An outbreak investigation follows an ordered sequence: prepare, confirm there are more cases than expected, verify the diagnosis, define a case, find cases, describe them by person, place and time, form and test hypotheses, compare with laboratory and environmental findings, apply control measures, keep up surveillance and communicate. The steps overlap: control measures start as soon as there is a likely source, without waiting for the analysis to end.
Worked example
- Data (made up): a health centre sees 18 cases of bloody diarrhoea in one week; 2 are expected.
- Confirm: 18 / 2 = 9 times the expected; the laboratory finds the same agent in 10 of 12 samples.
- Case definition: bloody diarrhoea since 1 May in residents of the village; active case finding turns up 31 cases.
- Describe: the curve shows a continuous peak and the map clusters cases near a public water tap.
- Test (case-control): 25 of 31 cases and 15 of 60 controls drank from the tap.
- OR = (25 × 45) / (6 × 15) = 1 125 / 90 = 12.5.
- Compare: a water sample from the tap shows faecal contamination and no residual chlorine.
- Act: the tap is closed, the water chlorinated, SIVIGILA notified, and case finding continues for two incubation periods.
- Reading: drinking from the tap is linked to 12.5 times the odds of illness; new cases fall to 0 in 10 days and the report is shared.
The steps grouped into four phases
| Phase | Steps | Output |
|---|---|---|
| Confirm | Prepare for field work, establish that there is an outbreak, verify the diagnosis | Confirmed outbreak |
| Describe | Define a case, find cases, describe by person, place and time | Curve, map, table and hypotheses |
| Test | Analytic study, refine hypotheses, compare with laboratory and environment | Source and route of spread |
| Act | Control measures, ongoing surveillance, communication | Outbreak controlled and report |
Markers
- Case definition
- Active case finding
- OR
- SIVIGILA
In practice
- The clinician is often the first sensor: reporting an unusual cluster on time is the first step of any investigation.
- Communication is part of the method: to the community, to the authorities and, at the close, a written report that helps prevent the next outbreak.
Common pitfalls
- Skipping confirmation: a rise in cases may come from more testing or a change in reporting.
- Looking only at the ill: without a comparison group the association cannot be measured.
- Waiting for the final result to act: the likely source is controlled at once.
Test yourself
Think of the answer, then tap the question to see it.
What is the first step after preparing to investigate a possible outbreak?
Establish that it exists: compare observed with expected cases and rule out reporting or diagnostic artefacts.
What goes into an outbreak case definition?
Clinical criteria (and laboratory criteria if applicable) plus limits of person, place and time.
Which analytic design is used when the outbreak happens in a closed, known group, such as party guests?
A retrospective cohort study: the attack rate is computed in those exposed and not exposed to each food.
Must laboratory confirmation of the source come before control measures?
No: measures start as soon as there is a likely source, while the investigation continues.
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 6, Section 2 (Centers for Disease Control and Prevention, 2012); SIVIGILA (Instituto Nacional de Salud, 2026).
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Representative model, for educational purposes only.