Pelvic inflammatory disease (PID) has been described in the medical literature for more than a century as a specific entity. Neisseria gonorrhoeae, Chlamydia trachomatis, and Mycoplasma hominis, along with Bacteroides fragilis and other anaerobic bacteria, have been most frequently associated with PID. Factors affecting the occurrence of PID have been extensively studied during the past two decades and include number of sexual partners, age, race, socioeconomic status, education, and contraceptive method. As knowledge concerning factors that contribute to PID increases, epidemiologic studies addressing such issues must become increasingly sophisticated, and the literature needs to be re-evaluated in light of present knowledge. Various risk factors for PID, types of epidemiologic studies, methods for conducting such studies, and data interpretation are reviewed.

Some authors have suggested that use of IUDs (intrauterine contraceptive devices) in nulliparous women should be contraindicated because of the risk for PID (pelvic inflammatory disease); others have found nulliparous women to be suitable candidates for IUD use. Clinical studies of copper IUDs in which parous and nulliparous women have been compared with regard to risk of PID often point to young age as a risk factor, regardless of parity. Vital statistics of PID rates also show this trend. An overall PID rate of 1.49/100 woman-years (WY) was calculated from data contained in 28 of the clinical studies reviewed in this paper. These studies included 25,674 women, of whom 42.5% were nulliparous. Studies of fertility following use of copper IUDs indicate that fertility is not impaired by their use.