What is the primary cause of Q fever?

The primary cause of Q fever is Coxiella burnetii, a highly infectious bacterium. This organism is primarily associated with livestock, particularly cattle, sheep, and goats. Q fever is often contracted through inhalation of contaminated aerosols, direct contact with infected animals, or consumption of unpasteurized dairy products.

Coxiella burnetii is notable for its ability to survive in harsh environmental conditions, which contributes to its transmission potential. The organism can exist in soil and is resilient against extreme temperatures, making it a significant public health concern in areas where livestock farming is prevalent.

In contrast, the other options listed are associated with different diseases. Histoplasma capsulatum is a fungus that causes histoplasmosis, Coccidioides species are responsible for coccidioidomycosis (Valley fever), and Aspergillus relates to a variety of respiratory illnesses and allergic reactions, including aspergillosis. Hence, Coxiella burnetii uniquely identifies Q fever's causative agent among the given choices.