**3.1 Epidemiology**

Chlamydia trachomatis (CT) is one of the most reported diseases; however, case reports likely underestimate the burden of disease because most infections are asymptomatic and are neither diagnosed nor reported. Case report data are strongly influenced by screening activity, for these reasons, case report data are not reliable indicators of either population incidence or population prevalence [16]. In 2016 the estimated global prevalence of CT was 3.8% in women (95% UI: 3.3–4.5) and 2.7% in men (95% UI:1.9–3.7) in the group from 15 to 49-year-old [17]. A total of 1,758,668 cases of CT infection were reported in 2018 in the United States. Among females aged 15–24 years, the cases reported by chlamydia screening, increased 11.8% from 2014 to 2018; in men, the cases increased 37.8% from 2014 to 2018; this may reflect an increased number of men being tested and diagnosed due to increased availability of screening tests; this could also reflect increased transmission. Among sexually active women aged 16–24 years, CT screening has been increased [3].

## **3.2 The pathogen**

CT is a gram-negative obligate intracellular bacterium; humans are its exclusive natural host. CT serovars include Agents of preventable blindness (serovars A–C), the most common bacterial sexually transmitted infections worldwide (serovars D–K), and lymphatic system infections (serovars L1–L3). Some distinctive features include its ability to avoid destruction by the host's innate and adaptive immune system. By autophagy, CT migrates to the upper genital tract and establishes a chronic infection. Without treatment, up to 50% of infected women continue to be infected for greater than 1 year [18, 19].

The life cycle of CT consists of 2 main phases, the elementary body (EB) and the reticulate body (RB). EBs are present in the semen from infected males and are also released from infected female genital tract epithelial cells. The EB represents the infectious extracellular form and the RB is the non-infectious replicative form; RBs can convert back to EBs as required. CT can enter the 3rd stage when it is exposed to certain stressors, like interferon-gamma, penicillin, or iron-depletion, the organism is metabolically active but does not divide and continues to increase in size [20].
