## Core Concept
The case fatality rate (CFR) is a measure used in epidemiology to describe the severity of a disease. It is defined as the proportion of deaths within a designated population of people with a particular condition over the course of the disease. The CFR is often expressed as a percentage and is a critical indicator of the lethality of a disease.

## Why the Correct Answer is Right
The correct answer states that CFR is not a fixed property of a disease. This is because the CFR can vary significantly depending on factors such as the quality of healthcare, the virulence of the pathogen, and the demographics of the affected population. For example, diseases like rabies have a nearly 100% CFR if not treated promptly, while others like influenza have a much lower CFR.

## Why Each Wrong Option is Incorrect
* **Option A:** CFR is a measure of disease severity. This statement is actually true, making it an incorrect choice as the false statement about CFR.
* **Option B:** CFR is expressed as a percentage. This is also true, as the CFR is often represented as a percentage, making it incorrect as the false statement.
* **Option D:** CFR can vary depending on healthcare quality. This statement is true and highlights one of the reasons why CFR is not a fixed property of a disease.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the case fatality rate is not constant for a disease; it can change based on various factors including improvements in medical care. For instance, the CFR for HIV/AIDS has decreased over the years due to advancements in antiretroviral therapy.

## Correct Answer: D.