## Definition and Distinction **Key Point:** Incidence and prevalence are complementary but distinct measures of disease frequency that answer different epidemiological questions. ### Incidence - Measures **new cases** occurring in a **defined time period** - Expressed as a rate (cases per person-years at risk) - Reflects the **force of morbidity** — the rate at which disease develops - Formula: $\text{Incidence} = \frac{\text{Number of new cases in a defined period}}{\text{Population at risk during that period}}$ ### Prevalence - Measures **all cases** (new and pre-existing) **at a point in time** - Expressed as a proportion (0 to 1, or percentage) - Reflects the **burden of disease** in the population - Formula: $\text{Prevalence} = \frac{\text{Total number of cases at a point in time}}{\text{Total population at that point in time}}$ ### Relationship Between Incidence and Prevalence **High-Yield:** The fundamental relationship is: $$\text{Prevalence} \approx \text{Incidence} \times \text{Average Duration of Disease}$$ This means: - A disease with high incidence but short duration (e.g., common cold) may have low prevalence - A disease with low incidence but long duration (e.g., diabetes) may have high prevalence - Both measures are needed for complete understanding of disease epidemiology **Clinical Pearl:** In chronic disease surveillance, prevalence is more useful for planning healthcare resources, while incidence is more useful for identifying causes and evaluating prevention programs. **Mnemonic:** **I-NEW, P-TOTAL** — Incidence captures NEW cases; Prevalence captures TOTAL cases at one point.
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