“Is your gut microbiome the key to health and happiness?” ran The Guardian headline in 2017. “Germs in your gut are talking to your brain,” declared the New York Times in January. Your microbiome—the ecosystem of bacteria, viruses, yeast, and other microbes living in your gut—may be sending signals to your brain that alter your mood, your behavior, and your nervous system’s vitality. But scientists are only starting to bring the microbiome-brain picture into focus.