Cancer starts when cells start to replicate out of control. However, the cells in question are still the correct cells for whatever organ or body structure the tumour is forming in, and that will remain the case until it gets so bad that it starts metastatizing to other parts of the body. Endometriosis, though, is where cells that *aren’t* supposed to be there (uterus wall cells in this case, as I understand it) start growing in odd places around the body. While I’m sure it’s a horrifically painful thing, I’m also sure it’s far less likely to kill you than most cancers.
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