Surveillance and Epidemiologic Investigation
What to know
Surveillance is the ongoing, systematic collection, analysis, and dissemination of health data for action. Candidates must know the difference between passive (routine reporting) and active surveillance (trained staff seek cases), prospective vs retrospective, and incidence (new cases/time) vs prevalence (all cases at a point). Understand the definitions of an outbreak (more cases than expected), epidemic, endemic, and pandemic, and the steps of an outbreak investigation: verify the diagnosis, establish a case definition, find cases, describe by person/place/time (epidemic curve), form and test hypotheses, implement control, communicate.
Measures and traps
Know attack rate = (cases / population at risk) × 100, and that the population at risk excludes those already immune or not exposed. Understand relative risk (cohort) vs odds ratio (case-control). Traps include:
- Confusing incidence with prevalence.
- Using the wrong denominator (all patients vs exposed patients).
- Treating a single case as an outbreak.
- Forgetting that surveillance data must be acted upon, not just collected.
- Confusing a case definition (clinical + time + place + person) with a diagnosis.
Worked example / rule of thumb
In a 100-bed unit, 5 patients develop a surgical-site infection out of 50 operated patients: attack rate = 5/50 = 10%. If the same 5 cases are counted among all 100 patients, that denominator is wrong. Rule: always ask “who was truly at risk?” before dividing. For outbreak response, remember the mnemonic: Verify, Define, Find, Describe, Hypothesize, Control, Report.