Which statement best captures the difference between incidence and prevalence?

• A. Prevalence measures the rate of new cases; incidence measures existing cases.
• B. Incidence is the rate of new cases over a period; prevalence is the number of existing cases at a given time.
• C. They are the same.
• D. Incidence measures mortality.

Answer: (B)

Incidence and prevalence describe different ways of looking at disease in a population. The key idea is that incidence tracks new cases that arise over a defined period, while prevalence counts all existing cases at a specific moment (or over a period). Incidence is a rate, typically expressed as new cases per person-time or as a cumulative risk over the time interval, focusing on the flow of new occurrences. Prevalence is a proportion, the number of people with the disease at a given time divided by the total population, reflecting how widespread the disease is at that moment. Because prevalence depends on how long people live with the condition and whether cases resolve or lead to death, it can be high even with modest incidence if the disease lasts a long time, and low if people recover quickly. Incidence, by contrast, tells you about the risk of developing the disease and is not about how long someone remains affected. The statement that best captures the difference is that incidence is the rate of new cases over a period, while prevalence is the number of existing cases at a given time. The other options mix up these definitions or refer to mortality, which is a separate measure.