Pelvic inflammatory disease (PID) affects the upper reproductive organs of women, including the uterus, ovaries and, most commonly, the fallopian tubes. These organs can become inflamed when bacteria spreads through the cervix (opening that separates the uterus from the vagina) and into the uterus. The bacteria may occur naturally in the vagina or are introduced during sexual intercourse. Unlike infections in other body parts, which are usually caused by one organism, PID infections frequently include multiple bacterial organisms.

Normally, the mucous membrane of the cervix prevents bacteria or other organisms from moving into the uterus. When the cervix is infected or when its mucous membrane is diminished, it can allow bacteria to infect the uterus, fallopian tubes, ovaries or surrounding tissues. The infection may scar or inflame tissues or create a collection of pus called an abscess. PID can take days or months to develop after the initial bacterial infection.

PID is the most common cause of female infertility. During a woman’s monthly cycle, an egg is released by one ovary and travels through the fallopian tube to the uterus. For pregnancy to occur, the egg must be fertilized by sperm, usually in the fallopian tube, and become implanted in the uterus. When PID scars the tissues of any of these organs, the egg may be blocked and cannot be fertilized.

PID is also the most common cause of ectopic pregnancy. Scar tissue can prevent a fertilized egg from moving to the uterus and the egg implants and grows in the fallopian tube instead. Left untreated, the growing embryo can rupture the tube and cause massive abdominal bleeding.

According to the Centers for Disease Control and Prevention, more than 1 million women in the United States are diagnosed annually with PID. More than 80 percent of those cases are caused by sexually transmitted diseases. PID is responsible for 100,000 cases of infertility annually.