Prevalence

Prevalence is the proportion of a population that has a condition at a given time or during a specified period. It measures how widespread the condition is, rather than how often new cases arise.

Incidence: Incidence measures new cases of a condition arising in a population over a specified period. Unlike prevalence, incidence counts new cases rather than everyone currently affected.
Prevalence proportion: A ratio of people with a condition to the population being studied. This ratio is the basic calculation behind prevalence.
Disease burden: The health impact of disease on a population, including illness, disability, and premature death. Prevalence helps describe how many people live with a condition.
Cumulative incidence: The proportion of an initially disease-free population that develops a condition during a specified period. It estimates new-case risk, not the share already affected.
Population at risk: The people who could develop a particular condition during a defined period. The chosen population determines which cases belong in prevalence's denominator.
Case definition: A set of criteria specifying who counts as having a particular condition. Changing the criteria changes which people enter the numerator.
Healthcare planning: The process of organizing health services to meet a population's needs. The number of existing cases informs staffing, treatment capacity, and supplies.
Incidence rate: The rate of new cases per unit of person-time at risk. Its person-time denominator differs from prevalence's population denominator.
Point prevalence: The proportion of a population with a condition at one specific point in time. This is the time-specific form of prevalence.
Denominator (epidemiology): The population count used as the base of an epidemiological measure. A mismatched denominator can distort the estimated proportion.

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