Pelvic inflammatory disease (PID) is an infection of the female reproductive organs. The pelvis is in the lower abdomen and includes the fallopian tubes,  and uterus. The main cause of PID is through a sexually transmitted infection (STI) such as chlamydia, gonorrhea or mycoplasma genitalium. These bacteria usually only infect the cervix, where they can be easily treated with antibiotics. Pelvic inflammatory disease is a serious infection that develops when certain STDs or other infections aren't treated. It can cause chronic pain and infertility.