Q fever bacteria (Coxiella burnetii), computer illustration. Coxiella burnetii is an obligate intracellular bacterium, meaning that it cannot reproduce outside of its host cell. It is the cause of Q fever, and is thought to be the most infectious agent known, with just one bacterium causing disease. Q fever is a disease present in livestock that can be spread to humans by inhalation of contaminated particles. It causes flu-like symptoms, with fever, malaise, headache and nausea. It can progress to pneumonia, which can be fatal.