Prevalence

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<p>

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.<p> 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.<p> 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 a useful parameter when talking about long lasting infections, such as HIV, but incidence is more useful when talking about infections of short duration, such as chickenpox.
See also: rare disease