**Core Concept:** Case fatality rate (CFR) is a statistical measure representing the proportion of cases that result in death within a specific population affected by a disease. It is crucial for understanding the severity and potential impact of an infectious disease outbreak.

**Why the Correct Answer is Right:** The correct answer, C. the proportion of cases that result in death, accurately describes the case fatality rate. The CFR reflects the mortality burden of a disease within a population, which is essential for public health officials to assess the severity and plan appropriate responses.

**Why Each Wrong Option is Incorrect:**
A. Severity (number of cases): While severity is related to CFR, it is more broadly defined as the proportion of cases with severe illness, not just those resulting in death.
B. Mortality rate (number of deaths): Different from CFR, mortality rate is the number of deaths per 1000 people, whereas CFR is the proportion of cases resulting in death.
D. Impact on healthcare system: While the CFR is a measure of disease burden, it does not directly reflect the healthcare system's strain or capacity.

**Clinical Pearl / High-Yield Fact:** Case fatality rate (CFR) is an essential metric for understanding the severity of an infectious disease outbreak and can help guide public health decisions, such as resource allocation and epidemiological surveillance. However, it is important to consider other factors like severity and impact on healthcare system alongside CFR to gain a comprehensive understanding of the disease burden.

In summary, the correct answer, C, represents the proportion of cases that result in death, and the other options are related but distinct concepts.