**Core Concept**
Plague is a zoonotic infection caused by the bacterium *Yersinia pestis*, primarily transmitted to humans through the bites of infected fleas or direct contact with infected animals. The disease has three main forms: bubonic, pneumonic, and septicemic.

**Why the Correct Answer is Right**
The correct answer is related to the primary mode of transmission of plague. In the bubonic form of plague, the bacteria are transmitted to humans through the bites of infected fleas, which live on rodents and other small mammals. The fleas ingest the bacteria while feeding on the blood of an infected animal and then transmit it to humans through their bites. This mode of transmission is responsible for the majority of plague cases.

**Why Each Wrong Option is Incorrect**
**Option A:** Direct contact with infected animals is a mode of transmission, but it is not the primary method by which plague is transmitted to humans.
**Option B:** Aerosol transmission is a mode of transmission for pneumonic plague, but it is not the primary mode of transmission for bubonic plague.
**Option C:** Waterborne transmission is not a recognized mode of transmission for plague.

**Clinical Pearl / High-Yield Fact**
Plague is endemic in many parts of the world, particularly in rural areas of the western United States, Africa, and Asia. It is essential for healthcare providers to be aware of the clinical presentation of plague, especially in patients who have recently traveled to or live in areas where the disease is endemic.

**Correct Answer:** D.