What does infectivity mean?
Infectivity is the degree to which an infectious agent, especially a pathogen, can spread from one host to another. It is commonly discussed in epidemiology when comparing viruses, bacteria, parasites, or other transmissible agents, and is often expressed through estimates of transmission rates or basic reproduction numbers. A pathogen may have high infectivity because it spreads readily between people, yet cause relatively mild disease; this separates infectivity from virulence, which measures severity, and from pathogenicity, which concerns whether infection leads to disease. The term is also used more broadly for the capacity of an agent to establish infection in a susceptible host. In public-health discussions, infectivity is considered alongside environmental stability, host susceptibility, and the duration of infectiousness when assessing how widely an infection may spread.
nounThe degree to which an infectious agent can be transmitted from one host to another. In epidemiology, it describes the transmissibility of a pathogen rather than the severity of the disease it causes.
- The degree to which an infectious agent can be transmitted from one host to another.
- The capacity of a pathogen to establish an infection in a susceptible host.
"Public health officials studied the infectivity of the new strain before recommending additional precautions."
"The study found that the variant's infectivity increased substantially in unvaccinated populations."
"Researchers modelled the infectivity of the parasite under different environmental conditions."
The plural is used mainly when comparing the transmissibility of different pathogens or strains.
"Researchers compared the infectivities of the two viral variants."
A pathogen can be highly infective without being especially virulent: it may spread easily while causing only mild illness.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does infectivity come from?
Infectivity was formed in the 19th century from the adjective infective or infectious plus the suffix -ity, which forms abstract nouns. Both elements ultimately trace to Latin inficere, meaning “to poison” or “to contaminate,” and the related infectio, meaning “a poisoning” or “infection.” The family later developed into English infection and infectious, and cognate forms appear in other Germanic and Romance languages.
Related word forms
How infectivity is actually used
This is primarily a technical term used in epidemiology, infectious-disease research, and public-health communication. It is distinct from virulence and pathogenicity, which concern harm and disease development rather than transmission.
Easily confused with infectivity
Infectivity concerns how readily an agent passes between hosts, whereas pathogenicity concerns the proportion of infected hosts that develop disease.
Infectivity measures transmissibility, while virulence measures the degree of harm or severity caused by an infection.