The factor that is NOT typically listed as affecting the infectivity of infectious agents is "Mode of transmission required." While the mode of transmission is important for the spread of infectious agents, the term "infectivity" generally refers to the capacity of an agent to establish an infection in a host, rather than the specifics of how it is transmitted.
The other factors mentioned—"Ability to fight host’s natural defenses," "Adaptability during growth and multiplication," and "Stability outside of its host"—are all relevant to how effectively an infectious agent can infect and propagate within a host or across hosts.