Which statement best distinguishes incidence from prevalence?

• A. Incidence refers to new cases during a period; prevalence refers to existing cases at a point in time.
• B. Incidence refers to existing cases; prevalence refers to new cases.
• C. Incidence is about severity; prevalence is about frequency.
• D. Incidence is measured by DMFT; prevalence is DMFS.

Answer: (A)

Incidence measures new disease events that occur over a defined period in a population at risk, while prevalence is the proportion of people who already have the disease at a specific point in time. In other words, incidence is about the risk of developing the condition (new cases during a time frame), and prevalence is about how widespread the condition is at a snapshot in time (existing cases). This distinction also explains how prevalence can be influenced by how long the disease lasts—longer duration tends to raise prevalence even if new cases are steady. So, new caries lesions developing over a year represent incidence, whereas the proportion of people with caries at a survey date represents prevalence. The other statements either swap the concepts or refer to measurements not used to define incidence or prevalence.