Pelvic inflammatory disease (PID) is a condition that affects the female upper genital tract including the fallopian tubes, uterus, and ovaries. It is often a complication of an untreated sexually transmitted infection (STI). PID usually develops when an STI like gonorrhea or chlamydia is left untreated and develops into a bacterial infection. PID can cause chronic abdominal pain. Those who suffer from the disease often become infertile.