Martins de Barros & Busatto Filho date the first report in fiction of folie à deux to the Brazilian author Machado de Assis in 1879. ^{Reference de Barros and Filho1} I submit that the first fictional account of ‘shared delusions’ was by Miguel de Cervantes over 250 years before. Cervantes wrote Don Quixote de la Mancha in or around 1604, publishing the first part in 1605 and the second, a decade later.

In Don Quixote, the eponymous hero, we have a domineering and voluble fantasist driven ‘out of his wits’ by the undue influence of books of chivalry: ‘He so buried himself in his books that he spent the nights reading from twilight till day break and the days from dawn till dark; and so from little sleep and much reading, his brain dried up and he lost his wits.’ ^{Reference de Cervantes and Cohen2} His character is steeped in rich descriptions of grandiloquent and persecutory delusions, polymorphic hallucinations and cognitive blunting. Sancho Panza, his squire, whom he enlists as his companion for his travels, is described as ‘an honest man – if a poor man can be called honest – but without much salt in his brain-pan’. ^{Reference de Cervantes and Cohen2}

So we have a dominant Don Quixote, who has lost his reason, and a submissive, not so bright Sancho Panza, thrown together through much of their travels, creating a situation ripe for the development of folie à deux. And indeed we see a slow erosion of reason in Sancho Panza. He initially displays some resistance and skepticism to Don Quixote's delusions about windmills being monstrous giants or St Benedict's monks being a crew of wicked and diabolical ‘perfidious scoundrels’. But he increasingly becomes convinced of the veracity of Don Quixote's beliefs. One example should suffice, the example of the balsam of Fierabras. This is a concoction that Don Quixote claims he can make on the cheap. He tells Sancho Panza, ‘If ever you see me cut through the middle in some battle […] you have only to take the part of my body that has fallen to the ground and place it neatly and cunningly, before the blood congeals, on to the half that is still in the saddle, taking special care to make them fit exactly. Then you must give me just two drops of this balsam to drink and, you will see, I shall be as sound as an apple.’ ^{Reference de Cervantes and Cohen2} Sancho replies, ‘If that is so, from now on I renounce the governorship of the promised isle, and all I want in payment for all my good services is for your worship to give me the recipe for that marvelous liquor.’ ^{Reference de Cervantes and Cohen2}

By the end, Sancho Panza's descent into these fantastic delusions is complete. So much so that at his death bed, when Don Quixote regains a measure of lucidity and tries to persuade Sancho to see reason, Sancho Panza is completely insightless and unamenable.

We are not witness to the effect of the separation of Sancho Panza from Don Quixote, as the story ends before it. But apart from that, the description of folie à deux is complete in this wonderfully told tale.