At first, Elara was furious. “Idiota?” she scoffed, donning her gloves. “The microbiota is a masterpiece of co-evolution!”

The bacterium did nothing intelligent. It had no goals. It just ate, divided, and excreted butyrate. That butyrate, she knew, fed her colon cells. It reduced her Crohn’s inflammation. It made her feel, in a vague, whole-body way, calm.

It appeared on her desk at the Sorbonne one rain-slicked Tuesday. No return address. Just a plain, leather-bound volume with the unsettling title stamped in gold foil: El Libro es la Microbiota Idiota .

She closed the book. The title glowed one last time.