Quotes from Mikkel Borch-Jacobsen

Furthermore, the way in which the two (Freud and Jung) corresponds invoke Fliess's ghost leaves no doubt on the subject. They threaten each other with it between the lines, they frighten each other with it, and they do so because they know (but with a secret, esoteric knowledge that never goes beyond the bounds of the private correspondence) that Fliess had gone mad owing to his correspondence with Freud.