Incidence

Definition

The rate of new occurrences of a specified event—such as disease cases, parasite infections, or population arrivals—within a defined population and time period, typically expressed as a proportion or per-unit-time measure. In medical and veterinary entomology, incidence distinguishes new infections from existing prevalence and is essential for tracking vector-borne disease dynamics, assessing intervention efficacy, and modeling arthropod-borne pathogen transmission.

Etymology

From Latin 'incidere' (to fall upon, happen), via Middle French 'incidence'

Example

Following the introduction of Wolbachia-infected Aedes aegypti mosquitoes in a trial area, the incidence of dengue fever dropped from 12 cases per 1,000 person-years in control neighborhoods to 2 cases per 1,000 person-years, demonstrating the bacterium's ability to suppress viral transmission.

Related Terms

• Prevalence
• attack rate
• vectorial capacity
• basic reproduction number
• force of infection
• Epidemic
• Endemic

Usage Notes

Incidence measures new events only and requires a time component (e.g., 'cases per month'), whereas prevalence counts all existing cases at a snapshot in time regardless of when they began. Incidence rate (new cases per population-time) differs from cumulative incidence (risk, proportion affected over a period). In arthropod ecology, 'incidence' may also describe the proportion of samples or sites where a species occurs, though 'occupancy' or 'frequency' is preferred for this spatial usage to avoid confusion with epidemiological incidence.