Endometriosis is a condition in which cells similar to those in the endometrium, the layer of tissue that normally covers the inside of the uterus, grow outside the uterus. Most often this is on the ovaries, fallopian tubes, and tissue around the uterus and ovaries; however, in rare cases it may also occur in other parts of the body.
It can cause sever chronic pain and infertility.
The prevalence is hard to establish because it can only be definitely confirmed by surgical procedure - current estimates are that it probably affects around 10% of women at some stage of their lives.
It was first described (in the Western scientific literature) in 1860, and since then several theories regarding the development of the disease have been suggested - none of which has been accepted as a definitive explantion.