Epidemiology
One-Sentence Definition
Epidemiology is the study of how disease is distributed in populations and what determines that distribution.
Simple Explanation
Who gets sick, where, when, and why — and what changes if we intervene.
Detailed Scientific Explanation
Measures of frequency
- Incidence — new cases per population per time (risk of getting it)
- Prevalence — existing cases at a point in time (burden)
- Attack rate — proportion infected in a defined outbreak population
- Case fatality rate vs infection fatality rate — denominator matters enormously
Measures of transmission
- R₀ — average secondary cases in a fully susceptible population
- Rt — the same, given current immunity and behaviour; the number that guides action
- Serial interval and incubation period — set the pace of an outbreak
Study designs, weakest to strongest for causality: case report → cross-sectional → case–control → cohort → randomized trial. Case–control is the workhorse of foodborne outbreak investigation.
Outbreak investigation steps: confirm the diagnosis → confirm an outbreak exists → define a case → find cases → describe by person/place/time → generate hypotheses → test them → control measures → communicate. Genomics now enters at the “test hypotheses” step (Phylogenomics and Outbreak Typing).
Bias to watch: selection bias, information bias, confounding, and — in genomic epidemiology — sampling bias, which is usually the dominant problem.
Clinical Importance
- Pre-test probability comes from local epidemiology; the same symptoms mean different things in different settings
- Empirical therapy depends on local resistance prevalence
Research Importance
- Quantifying intervention effect; linking exposure to outcome
Diagnostic Relevance
- Sensitivity/specificity are properties of a test; PPV/NPV depend on prevalence — see Model Evaluation in Clinical Microbiology
AMR Relevance
- Surveillance networks quantify resistance trends and guide empirical guidelines
Related MOCs
Active Recall Questions
- Incidence vs prevalence — which changes with disease duration?
- Why is Rt more actionable than R₀?
- List the first four steps of an outbreak investigation.