In epidemiology, the prevalence of a health-related state (typically disease, but also other things like smoking or seatbelt use) in a statistical population is defined as the total number of cases of the risk factor 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. It is used as an estimate of how common a disease is within a population over a certain period of time. It helps physicians or other health professionals understand the probability of certain diagnoses and is routinely used by epidemiologists, health care providers, government agencies and insurers.

Suppose we define a as the number of individuals in a given population with the disease at a given time, and b as the number of individuals in the same population at risk of developing the disease at a given time, not including those already with the disease. Then, we can write the prevalence as

Prevalence may also be expressed in terms of subgroups of the population based on different denominator data.