Q fever: An infectious disease due to the bacterium Coxiella burnetii whose symptoms include fever, headache, malaise, and pneumonia (interstitial pneumonitis), but not rash. The Q stands for query because the cause of the disease was long a question mark. Q fever is a zoonotic disease and is contracted from cattle, sheep, and goats. Chronic Q fever (infection that persists for more than 6 months) may develop years after the initial infection and can lead to serious complications including endocarditis. Transplant recipients, patients with cancer, and patients with chronic kidney disease are at increased risk of developing chronic Q fever.