## Core Concept
The question pertains to the epidemiology of plague, a zoonotic disease caused by *Yersinia pestis*, which can manifest in bubonic, pneumonic, and septicemic forms. Understanding the indicators for the outbreak of plague is crucial for public health surveillance and response.

## Why the Correct Answer is Right
The correct answer, . , refers to the sudden appearance of a large number of infected fleas, rodents, or other vectors. A key indicator for the outbreak of plague is the presence of **bubonic plague** in a population, which is often signaled by the death of rodents and other small mammals. This is because the bacteria typically circulate among rodent populations, and an increase in rodent deaths can precede human cases.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because, although individual cases of plague can occur sporadically, they do not typically signal an outbreak.
- **Option B:** This option is incorrect as it does not directly relate to known indicators for plague outbreaks.
- **Option D:** This option is incorrect because, while secondary cases can occur, especially in pneumonic plague, they are not the primary indicator of an outbreak.

## Clinical Pearl / High-Yield Fact
A critical clinical pearl is that **bubonic plague** is characterized by the presence of **buboes**, which are swollen, painful lymph nodes, typically in the groin, armpits, or neck. Early recognition of these symptoms, coupled with a history of exposure to rodents or fleas, can lead to prompt diagnosis and treatment.

## Correct Answer Line
**Correct Answer: C. .**