Q fever bacteria (Coxiella burnetii), coloured scanning electron micrograph (SEM). These Gram-negative, spore-forming intracellular bacteria are the cause of Q fever, a rare livestock disease that can be spread to humans through inhalation of contaminated particles. It is considered the most infectious organism known, as just one bacterium is capable of causing infection. It causes flu-like symptoms including fever, headache and nausea. It can also lead to hepatitis, pneumonia or inflammation of the heart lining, all of which can be fatal. Magnification: x20,800 when printed 10 centimetres wide.