In
epidemiologyEpidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health research, and helps inform policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive...
,
infectivity refers to the ability of a
pathogenA pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
to establish an infection. More specifically, infectivity is a pathogen's capacity for
horizontal transmissionHorizontal transmission is the transmission of a bacterial, fungal, or viral infection between members of the same species that are not in a parent-child relationship....
that is, how frequently it spreads among
hostIn biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...
s that are not in a parent-child relationship. It is closely related to the concept of
incidenceIncidence is a measure of the risk of developing some new condition within a specified period of time. Although sometimes loosely expressed simply as the number of new cases during some time period, it is better expressed as a proportion or a rate with a denominator.Incidence proportion is the...
, which is the measure of infectivity in a population.
Infectivity has been shown to
positively correlateIn statistics, dependence refers to any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence....
with
virulenceVirulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...
. This means that as a pathogen's ability to infect a greater number of hosts increases, so does the level of harm it brings to the host.
A pathogen's infectivity is subtly but importantly different from its transmissibility, which refers to a pathogen's capacity to pass from parent to child.
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