Epidemiological measures generally tell us either how far a disease spreads or how severe it is once someone has it, and virulence is about severity, not spread.
- Case fatality ratio: This looks only at the group of people who already have the disease and asks what fraction of them died from it. Because it is calculated purely among the sick, it reflects how dangerous the disease itself is, which is the definition of virulence.
- Morbidity rate: This shows how many people in a population get the disease, which speaks to how easily it spreads, not how dangerous it is once caught.
- Proportional mortality rate: This shows what share of all deaths in a population come from one disease, which depends on how common other causes of death are too, not purely on the disease's own severity.
- Specific morality rate: This measures deaths in a defined subgroup relative to that subgroup's whole population, again mixing in how common the disease is, not just how deadly it is once contracted.
Because virulence is about severity among those already infected, and only the case fatality ratio measures deaths purely against confirmed cases, it is the correct measure.
Therefore, the correct answer is Case fatality ratio.