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Outbreak investigation
An outbreak investigation is an orderly process – confirm that there is an outbreak and a diagnosis, write a case definition, find cases, describe them by person, place and time, form and test a hypothesis – while starting control measures as early as possible, without waiting for the investigation to end.
Why it matters in the exam
The exam presents scenarios of diarrhoea among campers at a summer camp, students after a shared meal or a family, and asks for the first step, how to write a case definition, what type of outbreak an epidemic curve shows, and which food is responsible from a table of attack rates. The first step is to confirm that cases really exceed the expected number and that the diagnosis is correct. A case definition contains clinical and laboratory criteria, is restricted by person, place and time, and is sometimes graded into confirmed, probable and possible cases. A curve with one sharp peak suggests a point common source, and the incubation period lets you estimate the moment of exposure; a curve with successive waves suggests person-to-person spread. In a closed group, such as wedding guests, for each food the attack rate is computed among those who ate it and those who did not with the ratio between them, and the suspect food is the one with a large difference that most cases ate. When there is no defined group a case-control study is done. The investigation ends with recommendations and a report that spreads the lessons.
Key points
- The first step is to confirm an outbreak and a correct diagnosis.
- A case definition contains clinical and laboratory criteria, person, place and time.
- A curve with one peak suggests a point source; waves suggest person-to-person spread.
- In a closed group attack rates are compared for each food; otherwise a case-control study is done.
- Control measures are started as early as possible.
Summary table
| Step | What is done | Exam point |
|---|---|---|
| Preparation and verification | Confirm an excess over the expected and a correct diagnosis | A change in reporting or in the laboratory can mimic an outbreak |
| Case definition | Clinical and laboratory criteria, person, place and time | Graded as confirmed, probable and possible |
| Case finding and description | Line list, epidemic curve, map | Sharp peak – point source; waves – person to person |
| Hypothesis and testing | Cohort in a closed group, case-control when no group is defined | Attack rate and RR per food; OR in case-control |
| Environmental and laboratory studies | Samples of food, water, suppliers and cooks | Genetic matching of strains strengthens the link |
| Control and reporting | Removing food, isolation, prophylaxis, communication and a report | Control measures start as early as possible |
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Sources
- CDC, Principles of Epidemiology in Public Health Practice, 3rd ed., 2012 — Investigating an outbreak
- Gregg (ed.), Field Epidemiology, 3rd ed., 2008 — Conducting a field investigation
- Israel Ministry of Health, Division of Epidemiology, current — Guidance for district health offices on outbreak investigation
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.