CHLAMYDIA Top of the list is chlamydia. Chlamydia is one of the most common STIs worldwide, especially among young people aged 15-24. It is caused by a bacteria called Chlamydia Trachomatis, and spreads mainly through vaginal, anal or oral sex. Chlamydia is mostly asymptomatic, but when symptoms appear they include: - Pain during urination - Unusual vaginal or penile discharge - Pelvic pain or pain during sex - Bleeding between periods - Testicular pain (for men)