Depends on how you do it:
The cost of magic is damage to the body. (Dragonlance series: Raistlin)
Magic only comes to the ill. It’s the speed of burning through life.
Those who do deep magic trade something important to them, like longevity.
Magic burns grooves through the mind that the sudden absence thereof would mimic mental illnesses.
Thanks 
I have no idea how people get this disease. I think my initial idea was that it’s a genetic defect. People with this can’t infect other people and I don’t think other people can get it.
Honestly, haven’t thought too deeply about this disease yet 
I love your answers.
They are very straightforward. It’s just a matter of sticking to whatever you choose.
I admire that.
Anyone else?