**5.1 Epidemiology**

*Chlamydia* infection is the most common bacterial STI in the world and among STIs, only the prevalences of herpes and trichomoniasis are higher. *Chlamydia* infection is highest in sexually active young adults under 25 years of age. *Chlamydia trachomatis* causes 30-50% of nongonoccal urethritis in men and mucopurulent cervicitis in women. In men less than 35 years of age *Chlamydia* is the principal cause of epididymitis. Although there is no lasting immunity and re-infection is common, women do clear the infection faster with increasing age. Lymphogranuloma venereum (LGV) is an uncommon disease and relatively rare in developed countries. The disease is most common in sub-Saharan Africa and is also reported in areas of the Caribbean, Central America, and Southeast Asia and sporadically in developed nations.
