Coloured transmission electron micrograph (TEM) of Chlamydia pneumoniae bacteria inside a lung cell. The brown area at centre is an inclusion body containing bacteria (yellow) at different stages in their life cycle. Chlamydia species bacteria are unusual in that they are obligate intracellular parasites, living and reproducing only inside cells. Human infection with C. pneumoniae is common and most often asymptomatic. However, it can cause community-acquired pneumonia and laryngitis. Infection with C. pneumoniae has been shown to be a risk factor for developing atherosclerosis (fatty plaques) in blood vessels. Magnification: x13,000 when printed at 10 centimetres across.