Prevalence

The prevalence of a disease is defined as follows:

Prevalence =

Number of cases of a disease present

in a population at a specified time

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Number of individuals in the population

at that specified time

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.

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.