In my childhood, I read a short story about a man who lands on Mars and finds that Martians looked exactly like humans. When he told them he was from Earth, everyone thought he had a disease that makes him hallucinate. So he takes some Martians to see his space ship, and they think they caught that disease from him. He's later sent to a hospital/asylum where he meets martians who really had that disease. Unfortunately, I don't remember how it ended.
It seems to be the story The Earth Men, part of the Martian Chronicles bundle of short stories by Ray Bradbury. From Wikipedia: