**Core Concept**
Malta fever, also known as brucellosis, is a zoonotic disease caused by a bacterium transmitted to humans through contact with infected animals or contaminated animal products. The causative agent of Malta fever is a Gram-negative coccobacillus that can survive outside the host for short periods.

**Why the Correct Answer is Right**
The correct answer is **Coxiella burnetii**. This bacterium is highly infectious and can be transmitted to humans through the inhalation of aerosolized particles from infected animal tissues, such as placenta or aborted fetuses. **Coxiella burnetii** has a unique life cycle, with a persistent phase that allows it to survive within host cells, evading the immune system. This persistent phase is a key factor in the chronic nature of Malta fever.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because *Francisella tularensis* is the causative agent of tularemia, a different zoonotic disease.
* **Option B:** This option is incorrect because *Yersinia pestis* is the causative agent of plague, a highly infectious disease caused by the bites of infected fleas.
* **Option D:** This option is incorrect because *Borrelia burgdorferi* is the causative agent of Lyme disease, a tick-borne illness that presents with distinct clinical manifestations.

**Clinical Pearl / High-Yield Fact**
Malta fever is a classic example of a zoonotic disease that can be transmitted to humans through contact with infected animals or contaminated animal products. Practitioners should be aware of the risk of transmission and take appropriate precautions when handling potentially infected materials.

**Correct Answer: C. Coxiella burnetii**

✓ Correct Answer: B. Patients present with undulant fevers, sweating (often with characteristic foul, moldy smell sometimes likened to wet hay), and migratory ahralgia and myalgia