Pelvic inflammatory disease is an infection of the female reproductive organs that often occurs when sexually transmitted bacteria spread from the vagina to the uterus, fallopian tubes, or ovaries. There are three main classifications: endometriosis, adenomyosis, and endometrial hyperplasia. Endometriosis occurs when endometrial tissue grows outside the uterus, adenomyosis involves endometrial tissue growing into the uterine wall, and endometrial hyperplasia is an overgrowth of the endometrial lining. Symptoms vary depending on the specific condition but can include pelvic pain and menstrual irregularities. Treatment involves antibiotics, hormonal therapy, or in severe cases surgery to remove infected tissues.