A genus of micro-organisms which includes those responsible for NON-SPECIFIC URETHRITIS (NSU), ORNITHOSIS, PSITTACOSIS and TRACHOMA. Chlamydia trachomitis can be sexually transmitted by both men and women, and in developed countries is the most significant cause of NSU. Chlamydia and Neisseria gonorrhoea (see GONORRHOEA) are the major cause of PELVIC INFLAMMATORY DISEASE, which affects around 100,000 women a year in the UK, most of whom are under 25 years of age. Chlamydia does not usually cause symptoms unless it spreads to the upper genital tract; such spread, however, may cause miscarriage (see PREGNANCY AND LABOUR) or ECTOPIC PREGNANCY. The incidence of chlamydia has doubled in the past five years and the incidence of ectopic pregnancies has also been rising. The infection may well be the main preventable cause of ectopic pregnancy, one estimate being that no fewer than half of such pregnancies are linked to chlamydia infection – a figure that is probably much higher in young women. A preventive campaign in Sweden found that over 15 years, the incidence of ectopic pregnancies fell at the same rate as that of chlamydia diagnoses. Chlamydia infection responds well to antibiotic treatment with doxycycline or azithromycin but education of the public about this often ‘silent’ infection, coupled with screening programmes, would go a long way to reducing its incidence.