Measures of disease frequency
Also known as: morbidity measures
Measures of disease frequency are the epidemiologic statistics that quantify how much disease exists in a population. The two core measures are incidence, which counts new cases arising over a period, and prevalence, which counts all existing cases at a point in time.
Every frequency measure is a count placed over a defined population and time frame. Incidence captures new disease: cumulative incidence, or risk, divides the number of new cases during a period by the number of people at risk at the start, while incidence rate divides new cases by the total person-time contributed by those at risk. Incidence describes how quickly disease is developing, so it is the measure of choice for studying causes and for evaluating prevention.
Prevalence captures existing disease. Point prevalence divides all cases present at a moment by the total population; period prevalence uses all cases present at any time during an interval. Because prevalence counts people who acquired the disease at any time in the past and still have it, it reflects both how often disease occurs and how long it lasts. In a steady state, prevalence is approximately incidence multiplied by average duration — which explains why a treatment that extends survival without curing disease increases prevalence while leaving incidence unchanged.
Related measures address specific questions. Attack rate is a form of cumulative incidence used in outbreak investigation over a short, defined period. Mortality rate applies the same logic to deaths, and case fatality rate is the proportion of people with a disease who die from it. Prevalence is the practical measure for planning services and allocating resources; incidence is the measure for etiology. Prevalence also affects test interpretation, since positive predictive value rises as prevalence rises even when sensitivity and specificity stay fixed.
USMLE Step 1 tests these definitions quantitatively. Questions supply a small table or a short scenario and ask you to calculate incidence or prevalence, or to predict how a described change — a new screening program, a life-prolonging drug, an effective vaccine — would move each measure. Keep the denominators straight: incidence uses only people at risk, so existing cases are excluded.
Key takeaways
- Incidence counts new cases over a period; prevalence counts all existing cases at a given time.
- Cumulative incidence uses people at risk as the denominator; incidence rate uses person-time.
- Prevalence is roughly incidence times average disease duration in a steady state.
- Treatment that prolongs life without curing raises prevalence but not incidence.
- USMLE Step 1 questions ask you to calculate these measures and predict how interventions shift them.