In public health terms, what is the difference between incidence and prevalence as applied to an outbreak in a deployed setting?

• A. Incidence is total existing cases; prevalence is rate of new cases
• B. Incidence is rate of new cases over a period; prevalence is total existing cases at a point in time
• C. Incidence is the number of deaths; prevalence is the number of recoveries
• D. Incidence and prevalence are the same

Answer: (B)

The key idea is that incidence and prevalence measure different aspects of disease in a population during an outbreak. Incidence is the rate at which new cases appear over a defined period in a population at risk, so it tracks how quickly the outbreak is spreading. Prevalence is the total number of people who have the disease at a specific point in time (point prevalence) or over a period (period prevalence), reflecting the current burden on the system.

In a deployed setting, tracking incidence helps you see transmission dynamics and the effectiveness of control measures (are new cases rising or falling week to week?). Tracking prevalence tells you how many people are currently ill and therefore how much healthcare capacity, supplies, and patient care resources are needed at that moment. For example, 20 new cases in a week among 1,000 at-risk individuals shows incidence, while having 60 people ill on a particular day shows the point prevalence of 60/1,000.

The other descriptions blur these distinctions: incidence is not the total existing cases, and prevalence is not simply a count of deaths or recoveries. They are related but describe different facets of the outbreak.