This clinical fact sheet details pelvic inflammatory disease (PID), an infection of the uterus, fallopian tubes and ovaries. If it is not treated early, PID can cause scarring to the fallopian tubes. This can cause subfertility or complications in pregnancy. Untreated PID can also cause ongoing pelvic pain. PID is usually caused by sexually transmissible infections, most commonly chlamydia or gonorrhoea. In about 70% of cases the cause of PID is not identified.