infectivity infectivity in epidemiology, infectivity is the ability of a pathogen to establish an infection. more specifically, infectivity is a pathogen's capacity for horizontal transmission — that is, how frequently it spreads among hosts that are not in a parent–child relationship. the measure of infectivity in a population is called incidence. infectivity has been shown to positively correlate with virulence, in plants. 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 one organism to another.