Which metric reflects the risk of developing disease over a defined time period?

• A. Incidence
• B. Prevalence
• C. Attack rate
• D. Case fatality rate

Answer: (A)

Incidence reflects the risk of developing disease over a defined time period. It counts new cases that arise in a population at risk during a specific interval and expresses them as a proportion of the at-risk population, which is exactly what we mean by the probability that someone will become a case in that time frame. For example, if 1,000 people disease-free at the start are followed for one year and 30 develop the disease, the cumulative incidence is 30/1000 = 3% for that year.

Prevalence, in contrast, measures how widespread the disease is at a given moment or over a period—the total number of existing cases, not the risk of new disease. The attack rate is similar to cumulative incidence but is typically used in outbreak investigations for a defined group over a short period, whereas the broader question targets the general concept of risk over time. Case fatality rate looks at the proportion of cases that die, reflecting severity, not the chance of developing the disease.