what three indications suggests that Chief Bromden suffers a serious mental illness.

people assume that he's deaf and dumb because he doesn't speak/ he hallucinates

Chief Bromden is the story's narrator, at this point he cannot be trusted fully since his reliability is in question. For one thing, he is prone to hallucinations, most of which involve the Combine, a matrix which allows civil wars to erupt within the ward at the whim of a huge bureaucratic, unnamed “machine” government. The fog that descends is a metaphor for Bromden's lack of mental clarity, thickening whenever he becomes less stable, receding as he gains confidence. It also is significant that Bromden chooses to remain silent, representing the quiescent persons of society who relinquish their own voices when confronted with authority.