Virulence

Definition

The quantitative measure of a pathogen's capacity to cause disease or damage in a host, encompassing infectivity, pathogenicity, and severity of harm. In arthropod-borne disease systems, virulence describes both the pathogen's effects on the vertebrate host and, when applicable, fitness costs to the vector itself.

Etymology

From Latin virulentus, 'poisonous' or 'full of poison', from virus, 'slimy liquid, poison'.

Example

Wolbachia bacterial strains introduced into Aedes aegypti mosquitoes exhibit reduced virulence to the insect host while blocking replication of dengue virus, illustrating how virulence can be context-dependent and manipulated for biocontrol.

Synonyms

• pathogenicity (loose, often conflated)
• invasiveness

Related Terms

• Pathogenicity
• Infectivity
• transmissibility
• vector competence
• host-parasite coevolution
• disease ecology
• Biological control

Usage Notes

Virulence is often conflated with pathogenicity, but specialists distinguish them: pathogenicity is the qualitative ability to cause disease (yes/no), whereas virulence measures how much damage occurs (a spectrum). In vector biology, high virulence in the vertebrate host may enhance transmission if it increases vector biting rates, but virulence harming the vector itself typically reduces transmission—creating trade-offs studied in evolutionary epidemiology. The term applies broadly to microparasites (viruses, bacteria, protozoa) and macroparasites (helminths, some mites) affecting arthropods or transmitted by them.