Population-level study of disease distribution and determinants: incidence, prevalence, risk factors, outbreaks, intervention effects. Substrate of public health, pharmacovigilance, and health-policy decisions.
epidemiology
Incidence and prevalence
Incidence rate = new cases per person-time; prevalence = existing cases per population at a time point. Relation at steady state:…
Basic reproduction number R₀
Expected secondary cases from one primary case in a fully susceptible population. Outbreak grows if R₀ > 1; herd immunity threshold = 1 –…
SIR compartmental model
dS/dt = –βSI/N; dI/dt = βSI/N – γI; dR/dt = γI. Basic deterministic model of epidemics; R₀ = β/γ, final-size equation ln(S∞/S₀) = –R₀(1 –…
Case–control study
Retrospective design sampling on outcome: compare exposure histories of cases and controls. Estimates odds ratio OR = (a·d)/(b·c) with 2×2…
Cohort study
Prospective (or retrospective) follow-up of exposed vs unexposed groups; estimates incidence rate ratio directly. More valid for temporal…
Confounding and causal inference
A third variable associated with both exposure and outcome creates spurious association. Controlled via stratification, regression…
Herd immunity
Population-level protection of susceptibles when a sufficient fraction is immune — R_effective = R₀(1 – p) < 1 when p > 1 – 1/R₀. Enables…
Outbreak investigation
CDC 10-step method: confirm outbreak, define cases, find cases, describe (person/place/time), hypothesis generation, analytic study…