Endometriosis is a currently non-curable autoimmune illness that women are born with in which pieces of the endometrium end up outside of the uterus, and are found on surrounding organs. At onset of menstruation, these pieces of endometrium receive the same hormonal signal that the lining of the uterus does - and these pieces begin to break down and bleed like the lining of the uterus does. This in turn sets off the pain receptors on the surfaces to which these rogue pieces of endometrial lining are attached, and so a widespread inflammation as well as an immune response is triggered.This often leaves a young girl or woman in pain so serious that she can vomit, pass out, and/or require a trip to the hospital.
Read more here: http://www.livingwithendometriosis.org/what-is-endometriosis