Usually pelvic inflammatory disease (PID) represents a progression of a sexual communicated disease from the vagina/cervix up to the uterus and fallopian tubes (which reside in the pelvis). This is a very serious sometimes life threatening disease since abscess and more generalized infections can result. The fallopian tubes are particularly susceptible to damage from infections and as a results a complication of PID is infertility due to scarring of the tubes and the inability to effectively transfer the egg for fertilization or, if able to fertilize, a predisposition for tubal pregnancies.
This is one of the main reasons (aside from preventing transmission to others) that chlamydia/gonorrhea and other STD's should be promptly and effectively treated.