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. 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.
References
Infectivity Wikipedia(Text) CC BY-SA