Which statement correctly defines incidence and prevalence with an example?

• A. Prevalence is the number of new cases in a defined period; Incidence is the total number of existing cases at a point in time
• B. Incidence is the number of new cases; Prevalence is the total existing cases
• C. Incidence measures mortality; Prevalence measures recovery
• D. Both refer to hospital admissions only

Answer: (B)

Think of incidence and prevalence as two different ways of counting disease in a population: new cases versus all existing cases.

Incidence is about new cases that develop during a defined period. It tells you the risk of developing the disease over that time. For example, if 200 people in a town of 10,000 develop asthma in one year, the incidence is 200 new cases that year (or 2% of the population if you express it as a proportion).

Prevalence is about all existing cases at a specific moment (point prevalence) or over a period (period prevalence). It includes anyone who currently has the disease, whether they were diagnosed before or during that time. For instance, if on December 31 there are 1,500 people in the town who have asthma, the prevalence is 1,500.

So the statement that matches the definitions is: incidence represents new cases, while prevalence represents the total number of existing cases. The other options mix up these concepts or introduce unrelated measures (like mortality or recovery) or restrict to hospital admissions, which isn’t how these terms are defined.