Incidence versus prevalence

Incidence, usually given per 100,000 people per year and adjusted for age, is the measure for causes and prevention: it rises when a risk factor spreads (lung cancer after cigarettes) or when screening finds more cases (prostate cancer after PSA testing). Prevalence reflects both incidence and survival, so it grows as treatment improves, and is the measure for health-system planning and survivorship needs; there are now over 18 million cancer survivors in the United States. Lifetime risk (about one in two in high-income countries) is a cumulative incidence, and stage-specific incidence is what screening programmes aim to shift.