Incidence is one of two basic ways of describing the occurrence of a disease in a population (see also prevalence). Incidence is a rate, showing how many new cases of a disease occurred in a population during a specified interval of time (usually expressed as number of new cases per unit time per fixed number of people; e.g., number of new cases of cancer per 10,000 persons in one year). Because of the restriction to “new” cases, the population in which the incidence is measured is restricted to those who are susceptible to getting the disease during the observation period (e.g., for some infectious diseases, life-time immunity can be established and so those who had been immunized should be excluded from the population under study). This restricted population is typically called the “at-risk population” because they are at risk of getting the disease (as contrasted with those who already have the disease or have become immune to getting it). For this reason, incidence is often also called “risk.” Incidence is important in allowing researchers to study the impact of harmful exposures or preventative interventions on the occurrence of disease, because it doesn’t depend on the length of disease course or its fatality (problems with which the prevalence measure suffers).