Prevalence and Incidence

Measures of disease frequency are an important aspect of epidemiology. They establish a means for predicting what diseases are present in a population and provide an indi­cation of the rate at which they are increasing or decreas-

ing. A disease case can be either an existing case or the number of new episodes of a particular illness that is diag­nosed within a given period. Incidence is the number of new cases arising in a population during a specified time. It is determined by dividing the number of new cases of a dis­ease by the population at risk for development of the dis­ease during the same period. Prevalence is the number of people in a population who have a particular disease at a given point in time or period. It is determined by dividing the existing number of cases by the population at risk for development of the disorder during the same period. Inci­dence and prevalence rates always are reported as propor­tions (e.g., cases per 100 or cases per 100,000).