Epidemiology and Disease Surveillance
Epidemiology and Disease Surveillance
This domain tests how you measure disease frequency, compare groups, interpret study designs, and run surveillance or outbreak investigations. Questions often require choosing the right measure and avoiding denominator errors.
Core rules and concepts
- Incidence counts new cases divided by population at risk over time; prevalence counts all existing cases at a point or period. Prevalence = incidence × average duration under steady conditions.
- Attack rate is incidence in a defined outbreak cohort. Case fatality ratio = deaths from a disease divided by diagnosed cases; it is not a mortality rate.
- Surveillance is ongoing systematic collection, analysis, interpretation, and feedback for action. Passive surveillance relies on routine reporting; active surveillance seeks cases; sentinel surveillance uses selected sites.
- Outbreak steps: verify diagnosis, establish case definition, find cases, describe time-place-person, generate hypotheses, test with analytic study, control, communicate.
- Bias examples: selection, information, confounding. Association does not prove causation; consider temporality, dose-response, consistency, plausibility.
Worked example
At a picnic, 100 people attended. Of 40 who ate potato salad, 24 became ill; of 60 who did not, 6 became ill. Attack rate exposed = 24/40 = 60%; unexposed = 6/60 = 10%. Relative risk = 60%/10% = 6.0. The salad is strongly associated with illness, but the investigation still needs a case definition, food histories, and possibly stool testing before declaring cause.
Traps that cost marks
Do not use prevalence as a risk measure in a cohort. Do not call case fatality a population mortality rate. Do not present surveillance data as proof of causation. Always check the denominator and the time window.