I am unqualified to make an intelligent argument whether consciousness is simply or is not a brain disease. There is nothing simple about the question.
I consider that conscious beings, with enough consciousness about them to identify with oneanother, become unconscious masses, led by the nose, to perform as minions in actions of hate towards other(s), against nature and environment. One might argue that it is all within their nature to be so? However, with full consciousness, shouldn't we should work together for better future(s). There again, you and I may never agree what or which is a better future.
We may agree that consciousness is a multilayered (multi)function of the brain (perhaps causing or with some sick behaviour!?).

The question is kind of pointless, as no matter what the answer is, it makes no difference. People would not live their lives any differently.

It may have started out that way, but it has obviously had evolutionary advantages for those that possessed it, so it has become the default condition.

If it helps an organization to function and cope with its environment, how could it be called a disease?