**Core Concept**
The killing power of a disease is a measure of its virulence, which is the ability of a pathogen to cause disease and lead to death. Virulence is influenced by factors such as the pathogen's ability to invade and replicate within the host, evade the immune system, and produce toxins.

**Why the Correct Answer is Right**
The correct answer refers to the concept of **virulence**, which is a critical factor in determining the severity and outcome of an infectious disease. Pathogens with high virulence, such as certain strains of bacteria like *Staphylococcus aureus* or *Pseudomonas aeruginosa*, can cause severe disease and high mortality rates. This is often due to their ability to produce potent toxins, such as alpha-hemolysin or exotoxin A, which can disrupt host cell function and lead to tissue damage and organ failure.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the killing power of a disease. While "morbidity" refers to the incidence or prevalence of a disease, it does not capture the severity or lethality of the disease.
* **Option B:** This option is incorrect because "infectivity" refers to the ability of a pathogen to infect a host, but it does not necessarily predict the severity or outcome of the disease.
* **Option C:** This option is incorrect because "pathogenicity" refers to the ability of a pathogen to cause disease, but it does not necessarily predict the killing power or virulence of the disease.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that virulence is a key factor in determining the severity and outcome of an infectious disease. Understanding the virulence factors of a pathogen, such as its ability to produce toxins or evade the immune system, can help clinicians predict the likelihood of disease severity and inform treatment decisions.

**Correct Answer: D. Virulence**