Chlamydia trachomatis, an obligate intracellular human pathogen, is one of three bacterial species in the genus Chlamydia. C. trachomatis is a Gram-negative bacteria, therefore its cell wall components retain the counter-stain safranin and appear pink under a light microscope.

The inclusion bodies of Chlamydia trachomatis were first described in 1907, the Chlamydia trachomatis agent was first cultured in the yolk sacs of eggs by Feifan Tang et al in 1957.

Chlamydial infection. Advances in the diagnostic isolation of Chlamydia, including TRIC agent, from the eye, genital tract, and rectum.

C. trachomatis includes three human biovars:
 - serovars Ab, B, Ba, or C - cause trachoma: infection of the eyes which can lead to blindness and is prevalent in Africa
 - serovars D-K - cause urethritis, pelvic inflammatory disease, ectopic pregnancy, neonatal pneumonia, and neonatal conjunctivitis
 - serovars L1, L2 and L3 - lymphogranuloma venereum (LGV).
 - Many, but not all, C. trachomatis strains have an extrachromosomal plasmid.

Chlamydia species are readily identified and distinguished from other chlamydial species using DNA-based tests.

Most strains of C. trachomatis are recognized by monoclonal antibodies (mAbs) to epitopes in the VS4 region of MOMP. However, these mAbs may also cross-react with two other Chlamydia species, C. suis and C. muridarum.

C. trachomatis is an obligate intracellular pathogen (i.e. the bacterium lives within human cells) and can cause numerous disease states in both men and women. Both sexes can display urethritis, proctitis (rectal disease and bleeding), trachoma, and infertility. The bacterium can cause prostatitis and epididymitis in men. In women, cervicitis,pelvic inflammatory disease (PID), ectopic pregnancy, and acute or chronic pelvic pain are frequent complications. C. trachomatis is also an important neonatal pathogen, where it can lead to infections of the eye (trachoma) and pulmonary complications. Chlamydia trachomatis is the single most important infectious agent associated with blindness; approximately 600 million worldwide suffer C. trachomatis eye infections and 20 million are blinded as a result of the infection.