Prevalence or prevalence proportion, in epidemiology, is the proportion of a population found to have a condition (typically a disease or a risk factor such as smoking or seat-belt use). It is arrived at by comparing the number of people found to have the condition with the total number of people studied, and is usually expressed as a fraction, as a percentage or as the number of cases per 10,000 or 100,000 people. “Point prevalence” is the proportion of a population that has the condition at a specific point in time. “Period prevalence” is the proportion of a population that has the condition at some time during a given period (“12-month prevalence”, etc.), and includes people who already have the condition at the start of the study period as well as those who acquire it during that period. “Lifetime prevalence” (LTP) is the proportion of a population that at some point in their life (up to the time of assessment) have experienced the condition.
Prevalence estimates are used by epidemiologists, health care providers, government agencies, and insurers.
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?” and incidence answers “How many people per year newly acquire this disease?”.