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In epidemiology, the prevalence of a disease in a statistical population is defined as the total number of cases of the disease in the population at a given time, or the total number of cases in the population, divided by the number of individuals in the population.

For example, the prevalence of obesity among American adults in 2001 was estimated by the U. S. Centers for Disease Control (CDC) at approximately 20.9%. In plain English, "prevalence" simply means "extent", but in scientific English it 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 distinct from incidence, which is a measure of the number of new cases. Prevalence involves all affected individuals, regardless of the date of contraction; whereas incidence only involves individuals who have newly contracted the disease during a specified time interval.

To illustrate, a long term disease 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.

Lifetime prevalence (LTP) is the number of individuals in a statistical population that at some point in their life (up to the time of assessment) have experienced a "case" (e.g., a disorder), compared to the total number of individuals (i.e. it is expressed as a ratio or percentage). Often, a 12-month prevalence (or some other type of "period prevalence") is used in conjunction with lifetime prevalence. There is also point prevalence, the prevalence of disorder at a more specific (a month or less) point in time. There is also a related figure lifetime morbid risk - the theoretical prevalence at any point in life for anyone, regardless of time of assessment. (example: Synopsis of article on "How Prevalent Is Schizophrenia?" from Public Library of Science)

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• Incidence
• Rare disease
• Epidemiology glossary

• PlusNews, the UN's HIV/AIDS news service provides HIV prevalence rates for nearly 60 countries worldwide