Photomicrograph of Chlamydia grown in culture. The sample was taken from rhesus monkey kidney cells and stained with giemsa. The cell nuclei appear red and the infective 'elementary bodies' of the Chlamydia, which develop in 'blisters' in the cells, fluoresce green. Chlamydia is a very common sexually-transmitted pathogen, which is caused by a bacterium, Chlamydia trachomatis. It can cause pelvic inflamatory disease (PID) in women (but is 'silent' in the majority of patients of both sexes). It can also cause blindness.