**Core Concept**
Q fever is a zoonotic disease caused by a bacterium that primarily affects ruminants, but can be transmitted to humans through inhalation of aerosolized bacteria or contact with infected animals. The causative agent is a type of Coxiella, specifically Coxiella burnetii.

**Why the Correct Answer is Right**
Coxiella burnetii is a gram-negative bacterium that is highly infectious and can survive for long periods in the environment. Human infection typically occurs through inhalation of aerosolized bacteria from contaminated animal products, such as milk or wool, or through contact with infected animals. The bacteria then infect macrophages and replicate within the host, leading to a systemic infection.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct genus and species of the causative agent of Q fever.
**Option B:** This option is incorrect because it is a different type of bacterium that causes a different disease, not Q fever.
**Option C:** This option is incorrect because it is a type of virus, not a bacterium, and is not associated with Q fever.

**Clinical Pearl / High-Yield Fact**
Q fever is often asymptomatic or mild, but can progress to severe forms of the disease, including pneumonia, hepatitis, and endocarditis. It is essential to consider Q fever in the differential diagnosis of patients with unexplained fever or respiratory symptoms, especially in areas where the disease is endemic.

**Correct Answer: D. Coxiella burnetii.**