Incidence is the number of new cases of a disease divided by the total population at risk during a certain period. It is often expressed as numbers per million. See also prevalence

As an example, in 2007, in a county with a population of 31,182, 10 opiate users sought treatment for the first time in 2007. The incidence is the number of new cases treated divided by the county population, expressed per given number of the population, i.e., per 100, per 1,000, per 10,000, per 100,000 etc. The rate in this example may be calculated as follows: (10/31,182) x 100,000, which gives an incidence rate of 32 per 100,000 of the county population in 2007.