Which statement is true about incidence?

• A. It measures all existing cases at a point in time
• B. It measures the prevalence of disease
• C. It describes the number of new cases that develop over a specific period of time
• D. It counts only severe cases

Answer: (D)

Incidence measures the number of new cases of a disease that develop in a defined population during a specified period. It can be expressed as a cumulative incidence (the proportion of the at-risk population that becomes cases over the period) or as an incidence rate per person-time. It requires the population at risk and excludes those who already have the disease at baseline. This differs from prevalence, which describes all existing cases—new and pre-existing—at a given time (point prevalence) or over a specified period (period prevalence). The true statement about incidence is that it describes the number of new cases that develop over a specific period of time; counting only severe cases would not define incidence.