PID is usually caused by more than one kind of bacteria. The bugs most commonly involved are the sexually transmitted bacteria Neisseria gonorrhoeae and Chlamydia trachomatis. (Up to 40 percent of women with untreated chlamydia develop PID.) However, other kinds of bacteria generally accompany these germs. The infection usually starts in the vagina, then moves up the reproductive system through the cervix, into the uterus, up the fallopian tubes, and finally into the ovaries.

Generally, gonorrhea-associated infections start quickly with more severe symptoms than the ones caused by chlamydia. When chlamydia is the major culprit, symptoms tend to be milder and fewer, developing slowly over a period of months or years. In either case, abdominal pain frequently begins during or shortly after a menstrual period.