[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Epidemiology](https://durgical.com/en/epidemiology/) › [Surveillance and outbreaks](https://durgical.com/en/epidemiology/areas/outbreaks/) › Outbreak investigation

# Outbreak investigation

- Concept

Investigating an outbreak means confirming it, counting and describing cases, finding the source, cutting it off and sharing what was learned.

Updated Nov 5, 2026

## Key points

- First confirm the outbreak: compare cases with the expected and rule out artefacts (a new case definition, more testing, a new doctor who reports).
- The case definition has clinical and laboratory criteria plus limits of person, place and time; a case can be confirmed, probable or suspected.
- Descriptive epidemiology (epidemic curve, map, table by age and sex) generates the hypotheses.
- Hypotheses are tested with a cohort study (a closed group, such as a wedding) or a case-control study (an open population).
- Control does not wait: the likely source is dealt with while the investigation goes on.
- In Colombia, events of public health interest are reported to SIVIGILA.

## 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.

## Memory aid

Confirm, Count, Compare, Cut off, Communicate: five Cs so no step is skipped.

## 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.

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## Connections

## Related 6

- [Epidemic curve](https://durgical.com/en/epidemiology/epidemic-curve/)
- [Attack rate](https://durgical.com/en/epidemiology/attack-rate/)
- [Case-control study](https://durgical.com/en/epidemiology/case-control-study/)
- [Cohort study](https://durgical.com/en/epidemiology/cohort-study/)
- [Public health surveillance](https://durgical.com/en/epidemiology/public-health-surveillance/)
- [SIVIGILA: public health surveillance in Colombia](https://durgical.com/en/epidemiology/sivigila-public-health-surveillance-in-colombia/)

## Related conditions 2

- [Acute (viral) gastroenteritis](https://durgical.com/en/conditions/acute-gastroenteritis/)
- [Cholera](https://durgical.com/en/conditions/cholera/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

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](https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson6/section2.html) (Centers for Disease Control and Prevention, 2012); [SIVIGILA](https://www.ins.gov.co/Direcciones/Vigilancia/Paginas/SIVIGILA.aspx) (Instituto Nacional de Salud, 2026).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/outbreak-investigation/