A chronic disease has low incidence (new cases few per year) but high prevalence because people live many years with the condition. Describe how incidence, prevalence, and average disease duration are mathematically related and explain why prevalence remains high despite low incidence.

• A. Prevalence and incidence are unrelated measures by definition; duration has no mathematical effect on prevalence so high prevalence with low incidence is inexplicable.
• B. Prevalence ≈ Incidence × Average Duration; long duration explains why prevalence is high despite low incidence because many people remain diseased at the snapshot.
• C. Prevalence is always lower than incidence for chronic diseases, so a high prevalence with low incidence indicates calculation error rather than duration effects.
• D. Incidence equals prevalence divided by two regardless of duration; therefore long duration is irrelevant to prevalence values and cannot explain observed patterns.

Answer: (B)