Which scenario best illustrates the difference between incidence and prevalence?

• A. Five new cases among 100 people over a month; 50 existing cases at the end of the month.
• B. Fifty new cases among 100 people over a month; five existing.
• C. Both new cases and existing cases counted the same.
• D. No new cases occur; prevalence remains constant.

Answer: (A)

Incidence measures how many people develop a condition during a defined time in a given population, while prevalence is the total number of people who have the condition at a specific point in time (or over a period). In this scenario, five new cases occur among 100 people over a month, which shows incidence—the rate of new cases during that month. At the end of the month, there are 50 existing cases, which shows prevalence—the overall burden of disease in the population at that moment. This combination nicely separates the idea of new occurrences from the total number affected at a point in time, illustrating the difference between the two measures. Other options either mix those concepts or don’t clearly demonstrate the distinction between new cases and existing cases.