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:
- Number of new cases of a disease occurring in
- a populations during a specified period of time
- -------------------------------------------------- × 1,000
- Number of individuals at risk of developing the
- disease during that time period
- Number of new cases of a disease occurring in
To illustrate, a disease with a long duration that was spread widely in a community in 2002 will have a high prevalence in 2003 (assuming 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.