Pelvic inflammatory disease (PID) is a bacterial infection of the female reproductive system that often starts in the vagina and spreads upwards, infecting the uterus, fallopian tubes and ovaries. PID can be caused by a variety of bacteria, but is most commonly caused by sexually transmitted infections such as chlamydia and gonorrhea. PID is a serious condition that can cause infertility, ectopic pregnancy and chronic pelvic pain. Symptoms of PID include lower abdominal pain, fever, unusual vaginal discharge and pain during sex. Prompt treatment with antibiotics is important to prevent serious complications.