The incidence of disease is defined as the number of new cases of disease occurring in a population during a defined time interval. The number is useful to epidemiologists because it is a measure of the risk of disease.

• The incidence rate is defined as the incidence divided by the sum of the different times each individual was at risk of the disease.
• The incidence per 1,000 is defined as follows:

Including the number of individuals at risk in the denominator makes this measure the most common way to express incidence, although other coefficients such as 100,000 are often used.

Incidence and incidence rate are not to be confused with prevalence, which is defined as the number of individuals with a certain disease in a population at a specified time divided by the number of individuals in the population at that time. This measure differs from incidence in that it does not convey information about risk.

To illustrate, a disease with a long duration that was spread widely in a community in 2002 will have a high prevalence in 2003 (remembering that it has a long duration) but it might have a low incidence rate in 2003. Conversely, a disease that is easily transmitted but has a short duration may have a low prevalence and a high incidence. Incidence rate is useful for talking about diseases like chickenpox, which have a lifetime risk of almost one, since it is measured per unit time so can tell us when infections are likely to occur.