Difference between ovarian cyst and pelvic inflammatory disease?

One is infection: An ovarian cyst is a fluid collection in the ovary. It is usually due to ovulation, or less commonly a tumor. Pelvic inflammatory disease (pid) is an infection of the pelvic structures (cervix, uterus , fallopian tubes, ovaries). An ovarian cyst will usually resolve spontaneously. Pid requires antibiotic treatment. It can cause abscesses, which will appear on ultrasound, and can be serious.