Prevalence
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==Definition==
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.
Mathematically prevalence can be defined as follows
let a = the number of individuals in the population with the disease at a given time
let b = the number of individuals in the population without the disease at a given time
<math>Prevalence = \frac{a}{a + b}</math>
==Examples and Utility==
For example, the [http://www.cdc.gov/nccdphp/dnpa/obesity/trend/prev_reg.htm prevalence of obesity among American adults in 2001] was estimated by the U. S. [[Centers for Disease Control and Prevention|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 [[physician|physicians]] with the probability of certain [[diagnosis|diagnoses]] and is routinely used by [[epidemiology|epidemiologists]], [[health care]] providers, government agencies, and [[insurance]] companies.
Prevalence is distinct from [[incidence (epidemiology)|incidence]]. Prevalence is a measurement of all individuals affected by the disease at a given point in time, regardless of the date of contraction; whereas incidence is a measurement of individuals who contract 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 at a given point of [[2003]] (assuming it has a long duration) but it might have a low incidence rate during 2003. Conversely, a disease that is easily transmitted but has a short duration is spread widely during 2002 is likely to have a low prevalence at any given point in 2003 (due to its short duration) but a high incidence during 2003 (as many people develop the disease). As such, prevalence is a useful parameter when talking about long lasting diseases, such as [[HIV]], but incidence is more useful when talking about diseases 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: [http://medicine.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pmed.0020146 Synopsis of article on "How Prevalent Is Schizophrenia?" from Public Library of Science])
==See also==
{{Wiktionarypar|prevalence}}
*[[Incidence (epidemiology)|Incidence]]
*[[Rare disease]]
[[Category:Epidemiology]]
== External links ==
* [http://www.plusnews.org/country-profile.aspx PlusNews, the UN's HIV/AIDS news service provides HIV prevalence rates for nearly 60 countries worldwide]
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