What term describes the severity of disease and the strength of a pathogen's ability to establish an infection?

The term that best describes the severity of disease and the strength of a pathogen's ability to establish an infection is "virulence." Virulence refers to the degree of pathogenicity of an organism, which includes its ability to cause disease, the severity of the disease it produces, and its capacity to incite a strong immune response in the host. Highly virulent pathogens can cause severe disease in infected individuals, while less virulent strains might lead to milder infections or asymptomatic cases.

Pathogenicity, while similar, is a broader concept that relates to a pathogen's overall ability to cause disease, but it does not specifically address the severity or strength of the infection. Infectiousness pertains to how easily an infection can spread from one host to another, and contagiousness is typically used to describe pathogens that can be spread easily through direct or indirect contact. These concepts, while related, do not capture the specific idea of severity and strength of infection in the way that virulence does.