**Core Concept**
Q-fever is a zoonotic disease caused by a bacterium that infects animals, particularly cattle, goats, and sheep. The bacterium is transmitted to humans through inhalation of contaminated particles from the animal's tissues, often during handling of infected animals or their products.

**Why the Correct Answer is Right**
The bacterium responsible for Q-fever is *Coxiella burnetii*. This bacterium is highly infectious and can survive for extended periods in the environment, making it a significant risk for human exposure. The pathogenesis of Q-fever involves the bacterium's ability to infect macrophages and evade the host's immune system, leading to a range of clinical manifestations, from asymptomatic infection to severe pneumonia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Q-fever is not caused by *Francisella tularensis*, which is the bacterium responsible for tularemia.

**Option B:** This option is incorrect because *Brucella abortus* is the bacterium responsible for brucellosis, a different zoonotic disease.

**Option C:** This option is incorrect because *Yersinia pestis* is the bacterium responsible for plague, a different zoonotic disease.

**Option D:** This option is incorrect because *Bartonella henselae* is the bacterium responsible for cat-scratch disease, a different zoonotic disease.

**Clinical Pearl / High-Yield Fact**
Q-fever is often asymptomatic in the early stages of infection, but can progress to severe pneumonia, particularly in immunocompromised individuals. The diagnosis of Q-fever relies on serological tests, such as the indirect immunofluorescence assay.

**Correct Answer: C. Coxiella burnetii**.