**Core Concept**
The case fatality rate (CFR) is a measure of the severity of a disease, representing the proportion of individuals with a specific disease who die from that disease. It is a key indicator of a disease's virulence and is used to compare the severity of different diseases.

**Why the Correct Answer is Right**
The case fatality rate is calculated by dividing the number of deaths from a particular disease by the number of individuals diagnosed with that disease, usually expressed as a percentage. For example, if 100 people are diagnosed with a disease and 20 die from it, the CFR would be 20%. The CFR takes into account the number of cases and deaths, providing a more accurate representation of a disease's severity than other metrics.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the case fatality rate. While it may be related to disease severity, it is not a specific measure of it.
* **Option B:** This option is incorrect because it describes the mortality rate, which is the number of deaths from a disease divided by the population at risk. While related, it is not the same as the case fatality rate.

**Clinical Pearl / High-Yield Fact**
A high case fatality rate is often indicative of a highly virulent disease, which can be a major public health concern. For example, diseases with high CFRs, such as Ebola and rabies, require prompt and aggressive treatment to prevent widespread outbreaks.

**Correct Answer: D. Proportion of individuals with a specific disease who die from that disease.**