Infectivity

Definition

The capacity of a pathogen to invade a host, survive, and replicate; distinct from pathogenicity (severity of disease) and transmissibility (spread between hosts). In medical entomology, infectivity describes how effectively an arthropod vector or the pathogen it carries can establish infection in a vertebrate host upon contact.

Etymology

From Latin infectus (stained, tainted) + -ivus (tending toward) + -ity (quality of)

Example

The infectivity of Plasmodium falciparum sporozoites injected by Anopheles mosquitoes depends on both parasite density in salivary glands and the duration of blood-feeding; even brief probing may transmit sufficient sporozoites to initiate malaria infection.

Synonyms

• infectiousness

Related Terms

• Pathogenicity
• transmissibility
• Virulence
• vector competence
• inoculation rate
• parasite burden
• sporozoite load

Usage Notes

Often conflated with contagiousness (ease of spread) or virulence (harm caused). Specialists distinguish infectivity as the first step—successful establishment—while transmissibility concerns onward transmission. In vector studies, 'infectivity' may refer to the proportion of vectors carrying transmissible-stage parasites, whereas 'vector infectivity' to hosts describes the probability of transmission per bite. Contrast with 'infectious period' (duration of shedding).