When the immunologist says that pathogens possess infectivity, what is the immunologist explaining? Infectivity allows pathogens to:

The normal host defense mechanisms against infection include (1) normal skin and mucous membranes, (2) phagocytic system, (3) humoral immunity, and (4) cellular immunity. The compromised host is an individual who has one or more defects in these defense mechanisms. (Source: NIH.gov)