**1. Introduction**

The Chlamydiae, lacking cytochromes, unable to synthesize ATP, and are therefore obligate intracellular microorganism. By including both DNA and RNA, reproduction by binary fission, and being sensitive to antibiotics, they are classified as bacteria. Chlamydiae are gram negative and symbionts of diverse organisms ranging from human beings to amoebae. This family comprises 11 species that are pathogenic to humans or animals [1].

Some species that are originally animal pathogens, such as the avian pathogen *Chlamydia psittaci*, can be transmitted to humans [1, 2].

The main agents that infect humans and cause the widest range of diseases are *Chlamydia trachomatis* and *Chlamydia pneumoniae* [2, 3].

Strains of *C. trachomatis* are divided into three biovars: trachoma biovar, genital tract biovar, and lympho granuloma venereum biovar. These biovars are further subtyped by 15 serovars. The trachoma biovar (serovars A–C) is the leading cause of preventable blindness in developing nations, whereas the genital tract biovar (serovars D–K) is the most prevalent sexually transmitted bacterium [3]. As an absolute sexually transmitted pathogen, *C. trachomatis* is isolated more in developed countries. It is one of the most common sexually translated microorganisms in the world [4].

It must be noted that *C. trachomatis infection* may be asymptomatic in both sexes. This makes it harder to diagnose and manage infection control. This microorganism is the most common acute urethritis cause in sexually active young population. In acute urethritis due to *C. trachomatis*, cardinal findings like urethral discharge, itching and dysuria are so mild or sometimes do not exist [5].

Although its prevalence varies geographically, it accounts for 20–50% of nongonococcal urethritis cases and according to a study conducted in Turkiye, 84% of all urethritis cases are classified as nongonococcal [6].

*C. trachomatis* can cause epididymitis and male infertility in men.

The pathogen causes a wide range of diseases in women from acute cervicitis to pelvic inflammatory disease. The disease is usually asymptomatic, therefore, the patients unconsciously continue spreading the infection [7].

Another issue that makes the management of *C. trachomatis* infection harder is that it occurs most of the time as a coinfection with the other sexually transmitted pathogens [8]. Thus, early detection and management of *C. trachomatis* genital infections are very important [9].

For the time being, as culture and serological techniques are insufficient for diagnosis, with their high sensitivity and specificity, nucleic acid amplification tests like PCR are gold standard methods [10].

The World Health Organization estimated a prevalence of chlamydia at 4.2% (95% uncertainty interval: 3.7–4.7) worldwide among women aged 15–50 years in 2012. These figures correspond to approximately 131 million new cases of chlamydia annually (100–166 million). Ascending genitourinary infection may result in ectopic pregnancy, infertility, and chronic pelvic pain in some women [11].

The lymphogranuloma venereum biovar (serovars L1–L3) causes an invasive urogenital or anorectal infection.

Three Chlamydia species affect the eye by forming different types of conjunctivitis: *C. chlamydia*, *C. psittaci,* and *C. pneumonia*.

The ocular clinical manifestations of these three species are as follows [12]:

*C. trachomatis*: Trachoma, adult inclusion conjunctivitis, neonatal inclusion conjunctivitis, and lymphogranuloma venereum conjunctivitis.

*C. psittaci*: *C. psittaci* conjunctivitis.

*C. pneumonia*: *C. pneumonia* conjunctivitis.

*C. trachomatis* is the most frequent conjunctivitis causing one among all. Besides, it had been and still is, in endemic areas of the world causing trachoma and hereby blindness and low vision, which is a major problem of public health. Therefore *C. trachomatis* will be the prominent subject of this chapter.
