Prevalence

Definition

In epidemiology and disease ecology, the proportion of individuals in a defined population that are infected, infested, or affected by a pathogen, parasite, or condition at a specific point in time (point prevalence) or over a defined period (period prevalence). Expressed as a percentage, fraction, or rate per unit population. Prevalence differs from incidence, which measures new cases over time; prevalence reflects both disease acquisition and duration. In arthropod studies, prevalence is commonly used to quantify infection rates in vectors (e.g., malaria parasites in Anopheles mosquitoes), parasitism in host populations (e.g., mite loads on bees), or pathogen circulation in tick communities.

Etymology

From Latin praevalentia, meaning 'superiority' or 'dominance', later adopted into medical statistics to describe the dominant or common presence of a condition in a population.

Example

A survey of Ixodes scapularis ticks in New England found a 15% prevalence of Borrelia burgdorferi infection, meaning 15 of every 100 ticks tested carried the Lyme disease spirochete at the time of collection.

Related Terms

• Incidence
• abundance
• intensity
• parasite load
• vector competence
• basic reproduction number

Usage Notes

Prevalence is a static measure (a snapshot) and must be distinguished from incidence (the rate of new infections). In wildlife and vector studies, prevalence is often reported as 'minimum infection rate' when pooled samples are tested. Prevalence can be age-dependent or seasonally variable; longitudinal studies may distinguish point prevalence (single timepoint) from cumulative prevalence (ever infected). In arthropod parasitology, prevalence is sometimes conflated with 'prevalence intensity'—careful authors separate prevalence (proportion infected) from mean intensity (average parasites per infected host).