 Virulence

Virulence is the degree to which a pathogen causes damage or disease in a specified host and setting. It is a quantitative aspect of pathogenic behaviour, not simply a label meaning that an organism can cause an infection.^[ref]

A comparison needs an endpoint and controlled conditions. One experiment might compare tissue damage after equal exposure; another might compare survival across a range of doses. If two strains differ in the dose required to produce an outcome, that result concerns the tested host, route and conditions. It is not automatically a universal ordering for every species or exposure route.

This also separates damage from transmission. A pathogen can spread efficiently without causing the most severe disease, while a damaging infection need not transmit efficiently. Keeping the outcomes separate makes evolutionary and experimental claims about virulence testable.