Which measure reflects the number of new cases of an illness per population at risk in a given period?

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

Answer: (B)

The main concept here is distinguishing how we quantify disease in a population over time, specifically focusing on new disease events versus existing ones. The measure that reflects the number of new cases of an illness per population at risk in a given period is incidence. It captures the risk of developing the disease during that time frame and uses only those who are disease-free at the start as the denominator—the population at risk. This emphasis on new occurrences means incidence rises when more people who were not ill become ill, and it stays tied to the time period being studied.

Incidence is typically expressed as the number of new cases per a standard population unit, such as per 1,000 or per 100,000 people, within the specified period. By focusing on new cases, it helps clinicians and public health professionals gauge how quickly a disease is spreading or presenting in a population.

In contrast, prevalence counts all existing cases (both new and preexisting) at a given moment or over a period, so it reflects how long people live with the disease and how often cases are diagnosed, recovered, or resolved. Mortality refers to deaths, not new disease occurrences, and morbidity is a broad term that describes illness or disease burden but does not specify that it concerns new cases per time period.