Which statement best describes the difference between incidence and prevalence in epidemiology?

• A. Incidence measures all existing cases; prevalence measures new cases over a period.
• B. Prevalence only counts fatal cases.
• C. Incidence measures new cases over a period; prevalence measures all existing cases at a point in time or over a period.
• D. Incidence equals prevalence divided by time

Answer: (C)

Incidence and prevalence answer different questions about disease in a population. Incidence focuses on risk and new events: how many people develop the disease during a defined period, usually expressed as cumulative incidence (a proportion of those at risk who become cases over the period) or as an incidence rate (new cases per person-time). Prevalence, on the other hand, measures the burden of disease at a point in time or over a period: the total number of people currently living with the disease, including both new and pre-existing cases, divided by the population.

This statement is the best because it clearly distinguishes the two concepts: incidence tracks new cases over time, while prevalence counts all existing cases at a given time (or over a time span). The other options mix up the ideas: one describes incidence as counting all existing cases (that’s prevalence), another incorrectly suggests prevalence only counts fatal cases (it includes all living cases, regardless of outcome), and the last option presents an incorrect mathematical relationship (incidence is not simply prevalence divided by time; a rough connection exists in steady-state as prevalence ≈ incidence × average duration, but it’s not a division by time).