Chlamydia is a common sexually transmitted infection caused by the bacteria Chlamydia trachomatis. It can cause symptoms like painful urination and discharge but is often asymptomatic. While anyone can get chlamydia, those at highest risk are teens, people with multiple sex partners, and illicit drug users. Chlamydia is treated with antibiotics like azithromycin, doxycycline, or amoxicillin. Left untreated it can cause infertility in women. Prevention involves limiting partners, using condoms, and getting tested annually.