Virulence of a disease is indicated byProportional mortality rateSpecific mortality rateCase fatality rateIncubation period

The virulence of a disease is indicated by the Case Fatality Rate. This is a measure of the severity of a disease by defining the total number of deaths as a proportion of reported cases at a specific time. It provides an indication of the lethality of a disease in those who have been diagnosed with it.

Here's a step-by-step explanation:

1. Proportional Mortality Rate: This is the proportion of total deaths in a specified population, over a specified time period, that are attributable to a specific cause. It does not indicate virulence as it does not consider the total number of cases of the disease.

2. Specific Mortality Rate: This is the total number of deaths in a specified population due to a specific cause. Again, it does not indicate virulence as it does not consider the total number of cases of the disease.

3. Case Fatality Rate: This is the proportion of individuals diagnosed with a certain disease who die within a specified time period. This does indicate virulence as it considers the lethality of the disease in those diagnosed with it.

4. Incubation Period: This is the time between exposure to a pathogen and the appearance of the first symptoms. It does not indicate virulence as it does not consider the severity or lethality of the disease.