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."

Source : Wikipedia

It can cause severe chronic pain and infertility - and can also be dangerous if it affects any vital organs.

The prevalence is hard to establish because it can usually only be definitely confirmed by surgical procedures - and, on average, it takes 8 - 10 years to be properly diagnosed. Current estimates are that it probably affects around 10% of reproductive-aged women.

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 explanation.

Since the disease was defined, several different pathogenetic pathways have been considered, including retrograde menstruation, benign metastasis, immune dysregulation, coelomic metaplasia, hormonal disbalance, involvement of stem cells and alterations in epigenetic regulation, but the true pathogenesis of endometriosis remains poorly understood. The knowledge of the exact mechanism of the origin and progression of this disease is significant for the appropriate treatment.

Source : The Main Theories on the Pathogenesis of Endometriosis IJMS, Volume 24, Issue 5

Update June 2023

New research (ref.) from Nagoya University, Japan, suggests that some incidences of endometriosis may be associated with infections of a bacteria known as Fusobacterium . A causal link, however, has yet to be demonstrated in humans.