Prevalence
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The prevalence of a disease in a statistical population is defined as the ratio of the number of cases of a disease present in a statistical population at a specified time and the number of individuals in the population at that specified time. For example, the prevalence of obesity among American adults in 2001 (http://www.cdc.gov/nccdphp/dnpa/obesity/trend/prev_reg.htm) was estimated by the U. S. Centers for Disease Control (CDC) at approximately 20.9%. In plain English, "prevalence" simply means "proportion" (typically expressed as a percentage).
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. 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.