What is the fog? is it real?

The fog is not real; it is symbolic of the world in which the patients live since their world is clouded by medication and an inability to see the world as it is. This fog is actually safer for them; Chief Bromden especially visualizes the ward being invade by fog. When McMurphy comes, he tries to bring them out of the fog that covers their minds; in the end, he pays heartily for such an attempt.