The prevalence of a disease is defined as follows:
Prevalence =
Prevalence is not to be confused with incidence, which provides a measure of occurrences of a disease in a specified time interval. Prevalence involves all affected individuals, regardless of the date of contraction.
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.
Prevalence is useful because it is a measure of the commonality of disease. It helps physicians with the probability of certain diagnoses and is routinely used by epidemiologists, health care providers, government agencies, and insurance companies.