Incidence is described as the number of new cases during a defined period.

• A. It is the number of existing cases at a point in time.
• B. It is the proportion of population with disease after one year.
• C. It is the total number of deaths due to disease.
• D. It is the number of new cases during a defined period.

Answer: (D)

Incidence measures how many people develop a disease in a population during a defined time, reflecting the risk of becoming diseased over that period. It relies on counting new cases among those who are at risk at the start. The option stating the number of new cases during the period fits this idea exactly.

In contrast, the number of existing cases at a point in time is point prevalence, showing how widespread the disease is right now. The proportion of the population with disease after one year sounds like a prevalence measure over time rather than new occurrences. The total number of deaths is about mortality, not new cases of disease. So the only statement that captures new occurrences within a defined period is the number of new cases during that period. For example, if 1,000 people are at risk and 30 develop the disease in one year, the cumulative incidence is 30 per 1,000 people (3%), illustrating the concept.