**3. Overview of** *Chlamydia* **pathogenesis**

#### **3.1 Acute infections**

*Chlamydia* species cause widespread infections in humans. *Chlamydia trachomatis* serovars A–C are the leading cause of non-congenital trachoma and are the major cause of blindness and visual impairment in developing nations [7, 8]. *C. trachomatis* serovars D–K are considered the world's most common sexually transmitted pathogen causing disease in the genital tract and in men, are the primary cause of nongonococcal urethritis [7, 9]. Following vertical transmission through an infected birth *Persistence in* Chlamydia *DOI: http://dx.doi.org/10.5772/intechopen.109299*

canal, *C. trachomatis* serovars D–K cause neonatal conjunctivitis and pneumonia. Respiratory infection with *C. pneumoniae* causes an average of 10% of communityacquired pneumonia cases and 5% of bronchitis and sinusitis cases. In addition, avian strains of *C. psittaci* have long been known to cause zoonotic respiratory illness in humans [10]. The *C. trachomatis* lymphogranuloma venereum (LGV) biovar (serovars L1–L3) causes invasive urogenital or anorectal infection [11].

#### **3.2 Persistent and chronic Chlamydial infections**

*C. trachomatis* serovars D–K are responsible for about 15–40% of ascending upper genital tract infections leading to serious complications in women, such as salpingitis, pelvic inflammatory disease, ectopic pregnancy, epididymitis in men, and infertility in women and men [8]. *C. trachomatis* originating from the genital tract is also associated with reactive arthritis, which develops in 1-3% of patients after genital Chlamydial infection [9]. *C. pneumoniae*, which can also disseminate from the site of the initial infection, is linked to several chronic diseases, including asthma, atherosclerosis, arthritis, cardiovascular disease, and even late-onset Alzheimer's disease [12]. In addition, unresolved respiratory *C. pneumoniae* infection may contribute to the pathogenesis of chronic inflammatory lung diseases, such as asthma and chronic obstructive pulmonary disease [12]. Further, *C. trachomatis* impedes human papillomavirus (HPV)-induced mechanisms that maintain cellular and genomic integrity, and it may be linked to cervical cancer [13].

Clinical conditions associated with inapparent *Chlamydial* infections include asymptomatic urethritis in male individuals and cervicitis in female individuals, and silent pelvic inflammatory disease in female individuals [3, 14]. The clinical significance of persistent infection is associated with the reactivation of infection after weeks or months in individuals treated with antibiotics, and negative culture results for individuals with strong serological titers and epidemiological associations [3].
