In a year with 500 new cases in a population of 10,000 and 2,000 existing cases, which measure describes the number of new cases?

• A. Incidence
• B. Prevalence
• C. Prognosis
• D. Etiology

Answer: (A)

Incidence is about new cases that develop over a defined period in a population at risk. In this scenario, 500 people developed the disease during the year, so this number directly describes the incidence. Prevalence, by contrast, counts all existing cases (both old and new) at a given time, not just new additions. Prognosis refers to the expected outcome of the disease, and etiology means the cause. So the measure describing the number of new cases is incidence. If you wanted a rate, you’d consider the population at risk (the total population minus existing cases) and express 500 new cases relative to that at-risk group.