Which best defines disease incidence?

• A. Number of new cases over a time period in a population at risk
• B. Prevalence at a point in time
• C. Total population size
• D. Number of deaths

Answer: (A)

Incidence is the rate at which people who are initially disease-free develop the disease over a defined period. It counts only new cases and uses the population at risk during that time (for example, new cases divided by the number at risk at the start, or by person-time). This distinguishes it from prevalence, which measures all existing cases at a point in time (or over a period) relative to the total population, not just new ones. The total population size by itself isn’t incidence, since it doesn’t account for how many are at risk or how quickly new cases arise. Deaths describe mortality, not the occurrence of new disease. For example, if 20 new cases occur among 1,000 people at risk in a year, the incidence reflects those new events in that at-risk group over that period.