PID occurs when microorganisms ascend to the upper female genital tract from the vagina and cervix. The condition can result from infection with aerobic or anaerobic microorganisms. The microorganisms Neisseria gonorrhoeae and Chlamydia trachomatis are the most common causes because they most readily penetrate the bacteriostatic barrier of cervical mucus. Common bacteria found in cervical mucus include staphylococci, streptococci, diphtheroids, chlamydiae, and coliforms, including Pseudomonas and Escherichia coli. Uterine infection can result from any one or several of these microorganisms or may follow the multiplication of normally nonpathogenic bacteria in an altered endometrial environment. Bacterial multiplication is most common during parturition because the endometrium is atrophic, quiescent, and not stimulated by estrogen.

Those at risk for PID include people with any sexually transmitted infection or multiple sex partners. Conditions or procedures, such as conization or cauterization of the cervix, that alter or destroy cervical mucus, can allow bacteria to ascend into the uterine cavity. Any procedure that risks transfer of contaminated cervical mucus into the endometrial cavity by an instrument (such as a biopsy curette or an irrigation catheter) poses an increased risk of PID, as do tubal insufflation, abortion, or recent intrauterine device insertion. Infectious foci within the body, such as drainage from a chronically infected fallopian tube, a pelvic abscess, a ruptured appendix, or diverticulitis of the sigmoid colon increase the risk of PID, as do infection during or after pregnancy, cigarette smoking, multiparity, douching, and intercourse during menses.

Possible complications of PID may include potentially fatal septicemia from a ruptured pelvic abscess, pulmonary emboli, infertility, and shock.