Which term describes disease occurrences in a population during a defined period, distinct from prevalence?

• A. Incidence
• B. Prevalence
• C. Morbidity
• D. Mortality

Answer: (A)

The key idea is distinguishing new disease occurrences from overall disease presence. The term describing disease occurrences in a population during a defined period is about new cases arising in that window, usually expressed as a rate per population per time (for example, 5 new cases per 1,000 people per year). This is different from prevalence, which counts how many people have the disease at a specific point or over a period, including those who already began before the interval. Incidence focuses on new events, while prevalence reflects total existing cases at a moment in time. For example, if 50 people develop the disease in a town of 10,000 over one year, incidence is 50 per 10,000 per year. If at the end of the year there are 120 people living with the disease, prevalence is 120 per 10,000. Mortality relates to deaths, and morbidity can refer to illness in general, but neither captures the idea of new cases arising over a defined period like incidence does.