## **Core Concept**
The lethality of an acute infectious disease can be measured by assessing the proportion of individuals who die from the disease out of those who are infected. This involves understanding key epidemiological indices.

## **Why the Correct Answer is Right**
The case fatality rate (CFR) is a measure of the severity of a disease and is defined as the proportion of deaths within a designated population of people with a particular condition over the course of the disease. It is calculated as the number of deaths due to a specific disease divided by the number of individuals diagnosed with the disease, usually expressed as a percentage. The CFR is a useful index to measure the lethality of an acute infectious disease because it directly reflects the severity and outcome of the disease in terms of mortality.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is blank and cannot be evaluated.
- **Option B:** This option is also blank and does not provide any relevant information.
- **Option C:** Similarly, this option is blank and cannot be considered.
- **Option D:** This option is blank as well.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the case fatality rate (CFR) is particularly useful during outbreaks or epidemics to quickly assess the severity of the disease. It helps in planning healthcare resource allocation and in communicating the risk to the public. A high CFR indicates a more lethal disease.

## **Correct Answer:** C. Case fatality rate.