Francisella tularensis bacteria, coloured scanning electron micrograph (SEM). These Gram-negative intracellular bacteria are the cause of tularemia, or rabbit fever. As well as rabbits the bacteria can infect voles, muskrats and humans. Depending on the mode of infection it may cause an ulcerated skin disease, or a potentially lethal pneumonic disease. Treatment is with antibiotics. Magnification: x18,300 when printed at 10 centimetres wide.