Causes of Pelvic Inflammatory Disease
When the disease-causing organisms travel from the cervix to the upper genital tract, PID occurs. About 90% of all cases of PID are caused by untreated gonorrhea and chlamydia. Abortion, childbirth, and pelvic procedures are included as other causes. Possibilities that you are more likely to get PID:

Have a sex partner who has sex partners other than you
For birth control, use an intrauterine device (IUD)
Have more than one sex partner
Have had PID before
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Are age 25 or younger and sexually active
Have an STD and do not get treated