Chlamydia trachomatis is a common cause of acute pelvic inflammatory disease (PID) because it is a sexually transmitted intracellular bacterium that infects the female reproductive tract. It often ascends from the cervix to the endometrium, fallopian tubes, and surrounding pelvic structures, leading to inflammation and damage. Early infection is frequently asymptomatic, allowing the bacteria to cause significant pathology before treatment. Prompt identification and treatment are critical to prevent complications like infertility and ectopic pregnancy. Other bacteria can be involved but C. trachomatis is the most frequent etiologic agent in young sexually active women.

Reference: Harrison's Principles of Internal Medicine, Fauci et al., 20th Edition.