Distinguish between incidence and prevalence.

• A. Incidence is the number of new cases in a defined period; prevalence is the total number of cases, new and existing, at a point or period.
• B. Incidence is the total number of cases; prevalence is the number of new cases.
• C. Incidence and prevalence are the same.
• D. Incidence is only used for chronic diseases.

Answer: (A)

Incidence measures the risk of developing a disease by counting new cases that appear during a defined time in people who were disease-free at the start. Prevalence measures the overall burden of the disease by counting all people who have the disease at a specific point in time or over a defined period, including those diagnosed before and still living with it. The key difference is that incidence is about new occurrences, while prevalence is about all current cases.

For example, if 1,000 people are disease-free at the start of the year and 25 develop the disease during that year, incidence is 25 new cases per 1,000 people per year. If, on a survey at the end of the year, 40 people in the population have the disease (whether they were newly diagnosed or had it before and remain affected), the prevalence is 40 per 1,000, or 4%. Incidence and prevalence relate differently: incidence reflects risk, while prevalence reflects how long people live with the disease, so diseases with long duration can have higher prevalence even with modest incidence.