Prevalence measures all existing cases of a disease within a population at a given point in time or over a specified period. It includes both newly diagnosed cases and pre-existing cases that are still present. This makes prevalence a measure of the total disease burden in the population rather than a measure of the rate at which new cases arise. In public health and epidemiology, prevalence is especially useful for understanding how widespread a condition is and for planning healthcare services, allocating resources, and evaluating community health needs. It differs from incidence, which focuses only on new cases occurring during a specified time period. Because the question asks what prevalence measures, the correct answer is the one that captures all current cases, not just past or new ones. It also does not refer to potential future cases, since prevalence is based on observed disease status. Therefore, the best answer is all existing cases of a disease.