Incidence

The number of cases of disease having their onset during a prescribed period of time. It is often expressed as a rate (for example, the incidence of measles per 1,000 children aged 5–15 years during a specified year). Measuring incidence may be complicated because the population at risk for the disease may change during the period of interest due to births, deaths, or migration, for example. In addition, determining whether a case is new—that is, whether its onset occurred during the prescribed period of time—may be difficult. Because of these difficulties in measuring incidence, many health statistics are instead measured in terms of prevalence. (Also see Sources and Definitions, Prevalence.)