Is dyslexia a brain disease or a disease of a different organ?

Definitely brain: Dyslexia, or trouble with decoding written material, is a result of certain parts of the brain not processing information properly. Reading is a relatively recent requirement for survival in evolutionary terms, so it is not all that surprising that problems in that domain are so common. It can't be "fixed, " but a number of well-established interventions can help greatly to ease the problem.

Not eye, brain: The ability for researchers to relate brain function to structure in "readers" vs. "non-readers" showed not only that different parts of the brain are used for reading in children with dyslexia, but also that, after intensive phonology-based therapy for 9 months, the same children with dyslexia were using more of the "typical"areas of the brain for reading & had significant improvement.