**Core Concept**
Q fever is a zoonotic disease caused by Coxiella burnetii, a Gram-negative bacterium. The disease is primarily transmitted to humans through inhalation of aerosolized bacteria from infected animals, such as cattle, sheep, and goats. Q fever is characterized by its acute and chronic forms, with symptoms ranging from mild flu-like illness to severe pneumonia and endocarditis.

**Why the Correct Answer is Right**
Coxiella burnetii is highly resistant to environmental stressors and can survive for extended periods outside a host. When inhaled, the bacteria infect alveolar macrophages and evade the host's immune response by producing proteins that inhibit phagolysosome formation. This allows the bacteria to replicate within the host, leading to the development of Q fever. The disease can manifest as an acute illness, characterized by fever, headache, and fatigue, or as a chronic form, which may progress to endocarditis or other complications.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the primary mode of transmission of Q fever.
**Option B:** This option is incorrect because Coxiella burnetii is not transmitted through tick bites, which are a primary vector for other zoonotic diseases such as Lyme disease.
**Option C:** This option is incorrect because the bacteria do not primarily infect the gastrointestinal tract, but rather the lungs and other organs.

**Clinical Pearl / High-Yield Fact**
Q fever should be considered in the differential diagnosis of patients presenting with pneumonia or endocarditis, particularly those with a history of exposure to infected animals or contaminated environments. The disease can be challenging to diagnose, and laboratory confirmation may be necessary to establish a definitive diagnosis.

**Correct Answer:** D