Endometriosis is a disease in which tissue that normally grows inside the uterus grows outside it. The main symptoms are pelvic pain and infertility.Nearly half of those affected have chronic pelvic pain, while in 70% pain occurs during menstruation.Pain with sex is also common.ndometriosis is a disease in which tissue that normally grows inside the uterus grows outside it. The main symptoms are pelvic pain and infertility.