**2. Epidemiology**

*C. trachomatis* was first isolated in the genital tract in the 1950s. However, it was not assigned much importance until the 1990s because of the high asymptomatic rate in both genders [11]. Since the late 1990s, *C. trachomatis* has been the most frequently reported sexually transmitted disease in Europe and America. In 2017 in the United Kingdom (UK), 203,116 people were diagnosed with new *C. trachomatis* infections, while 44,676 were diagnosed with *N. gonorrhoeae* infections and 7137 were diagnosed with syphilis [12].

The actual number of people infected with *C. trachomatis* is believed to be much higher due to the fact that the disease is mostly asymptomatic and goes undiagnosed. Its prevalence is 1–40%, depending on the population [11]. In the UK, a screening of people aged 15–24 conducted by the National Chlamydia Screening Programme detected more than 126,000 genital chlamydia infections [12].

In 2018, the incidence of chlamydial urethritis was 381 per 100,000 men in the United States of America (USA), making *Chlamydia* the most commonly detected and reported urethritis pathogen. In the same study, the incidence of gonococcal urethritis was 213 per 100,000. Every year, 4 million Americans are affected by urethritis. About 600,000 of these are gonococcal, while 3 million are nongonococcal urethritis, half of which are caused by *Chlamydia* [9]. In one study, *N. gonorrhoeae* was detected in 127 (30%) of 424 male patients with acute urethritis, while *Chlamydia* was detected in approximately 143 (33%) [13]. The reported rate of urethritis increased by 36% between 2008 and 2018. This rising trend is also being observed in Europe according to a report from the European Centre for Disease Prevention and Control [14]. The rate of newly diagnosed chlamydial infections continues to rise. In a study conducted in the USA, the rate of newly diagnosed chlamydial infections increased by 4.7% between 2015 and 2016 [15].

#### *Chlamydia Infection from Androgical Perspective DOI: http://dx.doi.org/10.5772/intechopen.110045*

This rapid increase in the prevalence of *Chlamydia* is also related to technological advances in diagnostic methods since the 1990s. Advancements in polymerase chain reaction technologies in particular led to the development of nucleic acid amplification tests (NAATs) [16]. With this method, extracted *Chlamydia* DNA fragments can be replicated to achieve sufficient samples for colorimetric evaluation, increasing the applicability and sensitivity of the test [17]. These assays are more effective than culture-based methods and could be used more widely in screening and diagnosis, resulting in a higher *Chlamydia* detection rate. However, the high rate of asymptomatic infection causes problems in the evaluation of infected individuals in health centers, leading to delays in diagnosis and treatment [18]. Therefore, the actual prevalence is much higher [19].

In a similar study, it was determined that the prevalence of *Chlamydia* infection peaked between the ages of 16 and 24 and was comparable in men and women [20]. The prevalence of *Chlamydia* infection was reported to be highest in women between the ages of 15 and 30, while in men it is seen between the ages of 20 and 29 [19]. The prevalence decreases rapidly after the age of 30 [20].

In a meta-analysis spanning 9 European countries, the prevalence of *Chlamydia* infection was found to be 2.7%, and there was no statistically significant difference in prevalence between men and women [19]. *Chlamydia* has been detected at a higher rate in African-Americans than in whites [15].

In a study conducted by Sonnenberg et al. on the British population, the rate of *Chlamydia* infection was found to be higher in those with gonorrhea. This was attributed to the fact that people with risky sexual behavior are more likely to encounter different pathogens. For this reason, gonococcal and nongonococcal infections can often occur concurrently [14]. There may be two pathogens in nongonococcal urethritis. The association of *M. genitalium* and *C. trachomatis* is not uncommon [7]. In some studies, dual infection was identified in up to 10% of cases [21].

In a study by Newbernet et al., adolescents with sexually transmitted diseases had twice the risk of contracting HIV infection compared to those without. Similarly, those who had a previous genital herpes infection were at increased risk of chlamydial, gonorrheal, and human papillomavirus (HPV) infection [14]. Other risk factors for urethritis include having multiple sexual partners at the same time, insufficient condom use, and having more than three different partners who are homosexual or bisexual. In addition, alcohol and other drug use can increase urethritis rates by contributing to risky sexual behaviors in young people [14].
