any coccoid rickettsia of the genus Chlamydia, parasitic in birds and mammals, including humans, and causing various infections, esp. of the eyes, as trachoma, lungs, as psittacosis, and genitourinary tract, as urethritis or chlamydia.
Also calleda widespread, often asymptomatic sexually transmitted disease caused by Chlamydia trachomatis, a major cause of nongonococcal urethritis in men and pelvic inflammatory disease and ectopic pregnancy in women.