Against the Grain by Joris-Karl Huysmans

In the course of that singular malady which plays such havoc with races of exhausted vitality, sudden intervals of calm succeed the crises. Without being able to explain the reason, Des Esseintes awoke quite strong and well one fine morning; no more hacking cough, no more wedges driven with a hammer into the back of the neck, but an ineffable sensation of well-being and a delightful clearness of brain, while his thoughts became cheerful; and instead of being opaque and dull, grew bright and iridescent, like brilliantly coloured soap bubbles.

This lasted some days; then in a moment, one afternoon, hallucinations of the sense of smell appeared.