The ability of a pathogen to cause infection in a host is known as:

The ability of a pathogen to cause infection in a host is best defined as pathogenicity. Pathogenicity refers specifically to the capacity of a microorganism to cause disease, which includes its ability to invade the host, multiply, and cause damage, leading to illness.

Infectivity relates to how easily a pathogen can establish an infection; while it's an important aspect of infectious diseases, it does not fully capture the entire process of causing disease that pathogenicity encompasses. Virulence refers to the degree of damage a pathogen can cause to the host, which is a subset of pathogenicity that focuses more on severity rather than the mere ability to infect. Communicability addresses how easily a disease can be spread from one host to another, which is different from the pathogen's inherent ability to cause infection in the first place.

Therefore, pathogenicity is the most appropriate term to describe the overall capability of a pathogen to cause disease in a host.