<P> Prevalence estimates are used by epidemiologists, health care providers, government agencies, toxicologists, and insurers . </P> <P> Prevalence is contrasted with incidence, which is a measure of new cases arising in a population over a given period (month, year, etc .). The difference between prevalence and incidence can be summarized thus: prevalence answers "How many people have this disease right now?" or "How many people have had this disease during this time period?" and incidence answers "How many people per year newly acquire this disease?" </P> <P> In science, prevalence describes a proportion (typically expressed as a percentage). 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% . </P> <P> Prevalence is a term which means being widespread and it is distinct from incidence . Prevalence is a measurement of all individuals affected by the disease at a particular time, whereas incidence is a measurement of the number of new individuals who contract a disease during a particular period of time . 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 . </P>

The prevalence of a disease in a community is influenced by which of the following factors