Incidence

The frequency of an event over a certain time period (usually a year), often expressed as a raw number. Incidence, typically, refers to the number of new instances or experiences in a given year. For example, if the incidence of HIV infection in prisons has been increasing, it means that more new infections are being reported in recent years than had been reported in previous years. Incidence is best understood when contrasted with prevalence. Prevalence refers to the commonality of a condition or event. HIV infection might be prevalent among prison populations even if the incidence in any given year was low. Incidence refers to the risk of contracting a particular disease (or experiencing a particular event), while prevalence indicates how widespread it is.