What term describes the severity of a disease caused by a pathogen?

The term that best describes the severity of a disease caused by a pathogen is virulence. Virulence refers to the degree of pathogenicity, which indicates how harmful or severe a disease can be once a pathogen infects a host. It encompasses the ability of a pathogen to evade the host's immune system, reproduce within the host, and cause damage to host tissues, leading to disease symptoms.

Infectivity, while related, refers specifically to the ability of a pathogen to establish an infection in a host. Pathogenicity is a broader term that indicates whether a pathogen can cause disease, but it does not assess the severity of the disease itself. Immunogenicity pertains to the ability of a pathogen to provoke an immune response, but again, it does not directly relate to the severity of the disease caused by that pathogen. Thus, virulence is the most precise term to denote how severe the disease can be.