Epidemic

Definition

The rapid increase in disease cases within a host population above the expected baseline, typically affecting a localized geographic area or defined group. In entomological and epidemiological usage, the term specifies temporal clustering and elevated incidence rather than mere presence of a pathogen. Thresholds vary by disease: for arthropod-borne infections, an epidemic may be declared when reported cases exceed historical averages or specific attack-rate criteria, such as 15 cases per 100,000 population over two consecutive weeks for meningococcal disease. Distinguished from endemic (constant baseline circulation), pandemic (global spread), and outbreak (smaller, more limited surge).

Etymology

From Greek epidēmia 'prevalence of disease', from epi- 'upon' + dēmos 'people'

Example

A West Nile virus epidemic in a temperate city often follows hot, dry summers that shorten the extrinsic incubation period in Culex mosquitoes, causing a sharp spike in human neuroinvasive cases above the endemic baseline of sporadic summer infections.

Synonyms

• epizootic (when restricted to non-human animals)

Related Terms

• Endemic
• pandemic
• Outbreak
• attack rate
• extrinsic incubation period
• vector-borne disease
• Epizootic
• basic reproduction number

Usage Notes

Specialists distinguish epidemic from epizootic (animal populations) and epiphytotic (plant diseases), though 'epidemic' is sometimes used loosely for all three. Thresholds are disease-specific and context-dependent; entomologists working on vector-borne disease surveillance must track both human case incidence and vector infection rates to detect early epidemic signals. The term implies a quantitative threshold crossed, not merely 'many cases'.