## Core Concept
Plague is a bacterial infection caused by *Yersinia pestis*, which is typically transmitted to humans through the bites of infected fleas that live on rats and other rodents. This disease is a classic example of a zoonotic infection, which means it can be transmitted from animals to humans.

## Why the Correct Answer is Right
The correct classification of plague as a zoonosis is due to its natural reservoir in rodents and other small mammals, and its transmission to humans through vectors like fleas. *Yersinia pestis* circulates among these animal populations, causing outbreaks that can spread to human populations when the bacteria are transmitted through flea bites or direct contact with infected animals.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because while plague does have a natural reservoir, the specific classification as a zoonosis directly relates to its transmission dynamics between animals and humans.
- **Option B:** This option is incorrect as it does not accurately describe the nature of plague transmission or its classification as a zoonosis.
- **Option C:** This option might seem plausible but does not accurately represent the specific characteristics of plague as a zoonotic disease.

## Clinical Pearl / High-Yield Fact
A key point to remember is that plague can manifest in different forms, including bubonic, pneumonic, and septicemic plague, with the bubonic form being the most common. The disease is highly infectious and can cause severe illness if not treated promptly with antibiotics.

## Correct Answer: D. Zoonosis.