How do incidence and prevalence differ in surveillance?

• A. Incidence measures all existing cases; prevalence counts new cases.
• B. Incidence counts new cases over a period; prevalence counts all existing cases at a point in time.
• C. Incidence is about mortality.
• D. Prevalence equals incidence.

Answer: (B)

The main idea here is understanding what each measure captures in surveillance. Incidence is about new disease cases that arise in a defined population over a specified period. It tells you how fast the disease is spreading or how often new cases occur. Prevalence, on the other hand, is the total number of people who have the disease at a given moment (point prevalence) or over a defined period (period prevalence). It reflects the overall burden of disease in the population, including both new and existing cases, and is influenced by how long people stay ill.

Why this answer fits best: it separates new events from the existing pool. In practice, incidence is useful for gauging transmission and the effectiveness of prevention or control measures, while prevalence helps assess the current load on health resources and services. For example, a chronic disease with long duration can accumulate many existing cases, leading to high prevalence even if new cases are infrequent. Conversely, an acute illness may have a high number of new cases over a period but relatively low prevalence if people recover quickly.

The other statements mix up these concepts: incidence is not all existing cases, it’s new cases; mortality is about deaths, not incidence; and prevalence does not equal incidence since they measure different things.