**Core Concept**
The plague is a zoonotic disease caused by the bacterium *Yersinia pestis*, which can be transmitted to humans through the bites of infected fleas or direct contact with infected animals, such as rodents. There are three main forms of the plague: bubonic, septicemic, and pneumonic.

**Why the Correct Answer is Right**
The pneumonic form of the plague is the only form that can spread from person to person through respiratory droplets, making it highly contagious and potentially leading to outbreaks. This form of the disease occurs when the bacteria infect the lungs, causing pneumonia. The bacteria can then be transmitted to others through coughing, sneezing, or close contact with an infected person.

**Why Each Wrong Option is Incorrect**
**Option A:** Bubonic plague is the most common form of the disease, characterized by swollen lymph nodes, or "buboes," in the groin, armpits, or neck. However, it is not transmitted from person to person.
**Option B:** Septicemic plague occurs when the bacteria enter the bloodstream directly, causing fever, chills, abdominal pain, and bleeding into the skin and organs. Like bubonic plague, it is not transmitted from person to person.
**Option C:** This option is blank, so there is no incorrect answer to explain.

**Clinical Pearl / High-Yield Fact**
The pneumonic form of the plague is often the most severe and deadly form of the disease, with mortality rates as high as 90% if left untreated. Early recognition and treatment are critical to prevent outbreaks and control the spread of the disease.

**Correct Answer:** C. Pneumonic plague.