A 17-year-old girl presents to your gynecology clinic with complaints of recurring pelvic pain. A probable diagnosis of pelvic inflammatory disease is made. She is a senior at a suburban high school, is involved in orchestra, and has been on the honor roll most years. She does not do drugs. She has had an equally "squeaky clean" type of boyfriend since last fall, and 6 months ago they had unprotected sex on three occasions. What is the most likely causative agent?