[speaking slowly -- as if he's trying to translate things to words]
It happens when a part of the body changes so that it grows uncontrollably and in a way that 'crowds out' the normal functions of the body. If left alone, it will kill the host by disrupting vital functions of the body. And, yet, it came from the body itself -- a tiny mistake in the pattern of growth that wasn't corrected. It's not like an infectious disease that is spread by a virus or bacterium or parasite.
My people's healers can reverse the damage -- tell the tumor to die and be reabsorbed by the body and the healthy cells to repair the damage. Before we met humans, humans would remove the tumors by surgery or by burning them out by radiation, or using chemicals that were more lethal to the tumors than healthy parts of the body. They had to hope that the damage both the cancer and the cure did would be healed by time, or to accept that the damage was worth saving a life.
Removing a tumor won't fix the damage it causes, you're right. But it stops the progression of the disease.
It depends on what you think Mirror is -- whether the harmful parts of her are essential to her mind, or are things that used to be useful parts of her mind but have become tumors.