**Core Concept:** Q-fever is a zoonotic disease caused by bacteria from the Coxiella burnetii genus. It can affect humans and is transmitted via inhalation of infected aerosols from animals or contaminated environments.

**Why the Correct Answer is Right:** Coxiella burnetii is the causative agent of Q-fever. It is an obligate intracellular bacterium that can survive and replicate within macrophages. Humans become infected by inhaling contaminated aerosols from infected animals or environments. Q fever is a global disease, predominantly found in endemic regions like Europe, Australia, and parts of Africa and Asia.

**Why Each Wrong Option is Incorrect:**

A. Coxiella burnetii is not the correct answer as it is the causative agent of Q-fever, not the disease itself.
B. Q fever is not caused by a virus, it is bacterial in nature.
C. While Q fever can be transmitted via contaminated food, the correct answer should mention inhalation from infected animals or environments.
D. The statement about the disease being rare is incorrect, as Q fever is indeed a global disease with endemic regions.

**Clinical Pearl / High-Yield Fact:** Q fever may present with a wide variety of symptoms, including fever, headache, and malaise. Inhalation pneumonia is the most common presentation, followed by hepatitis, endocarditis, and arthritis. Diagnosis is typically made through serological testing, as C. burnetii infection can lead to a chronic phase with asymptomatic carriage. Treatment involves antibiotics like doxycycline or azithromycin, along with supportive care.

**Correct Answer:** D. Coxiella burnetii is the causative agent of Q-fever.