hlamydia as an STI is a disease without a history in two senses: firstly, it only became a clinical entity in the 1970s; and secondly, its medical and social history has still to be written – an absence this chapter will begin to correct. Its seeming novelty in the 1970s prompts the question: was it an infection newly arrived in humans, or had it always been present and was now being recognised for the first time? This is a question that can only be answered, not by a medical historian, but by investigations of past populations by epidemiologists, pathologists and, perhaps most likely, by archaeologists using genomics. However, medical publications and oral testimony reveal that doctors in the 1970s were confident that chlamydia was not a new disease. They believed this new ailment had actually been around for decades and probably centuries, being one, probably the most important, of the infections that had been hidden in that most unsatisfactory disease entity non-gonococcal urethritis (NGU), also called non-specific urethritis (NSU). NGU was defined by what it was not. It required a laboratory diagnosis and had been invented by clinicians when, in patients with gonorrhoea-like symptoms, the gonococcus —could not be found in genital discharges. NGU was unaffected by antibiotics used to treat gonorrhoea, which became further negative diagnostic confirmation. The laboratory was also crucial to the creation of chlamydia as a clinical entity, which came from the development of new techniques in the 1960s and 1970s which enabled doctors to differentiate NGU into component infections: C. trachomatis, lymphogranuloma venereum (LGV, which was later shown to be caused by different strains of C. trachomatis from those causing chlamydia), Mycoplasma genitalium and trichomoniasis (caused by the protozoan Trichomonas vaginalis).