In the Shadow of Young Girls in Flower (translated by James Grieve), pp. 71-82.
Concerned by his "attacks of breathlessness," the narrator's parents call in Dr. Cottard, who prescribes "Drastic, violent purgatives. Milk and nothing but milk for several days. No meat. No alcohol." Quite sensibly, they reject this seemingly absurd treatment, but when the narrator keeps getting worse, they give in and follow Cottard's advice. And it works. "So it was that we realized that Cottard the buffoon was a great doctor."
When he gets better, however, they are convinced that the air of the Champs-Élysées is "unhealthy," endangering his meetings with Gilberte. But to his surprise, even disbelief, Gilberte invites him to tea at her house.
In love, it is not only the causes of catastrophe that may lie forever beyond our grasp: just as often we remain in ignorance of the whys and wherefores of sudden outcomes that are happier -- such as the one that Gilberte's letter brought to me -- or, rather, outcomes which appear to be happy, as there are few truly happy outcomes in the life of a feeling, which can generally look for no better reard than a shift in the side of the pain it entails.
The invitation is an accident: The narrator's friend Bloch is present one time when Cottard is making a call and comments that Mme. Swann is very fond of the narrator -- which is, as far as either of them knows -- untrue. But Cottard, always on the lookout to ingratiate himself, apparently speaks favorably of him to Odette, and the narrator becomes a regular visitor, welcomed cordially by the Swanns -- who have apparently forgotten their previous animus toward him (if it ever existed).
Such is the narrator's adulation of the Swanns that, when he reports to his parents that the staircase in the Swanns' home is a Renaissance antique and his father replies that it's a copy in a commonplace building where he once considered renting an apartment, the narrator clings to his faith: "I exercised the authority of my inner self and, despite what I had just heard, put behind me once and for all, as a true Catholic might shun Renan's Life of Jesus, the corrosive notion that the Swanns' apartment was a perfectly ordinary apartment, an apartment that we ourselves might have lived in."
And so the narrator is swept up in his adulation of the Swanns and his passion for Gilberte.