You have more microbial cells in your body than you have human cells. There are trillions of microbes that reside in your gastronistestinal tract called your gut microbiome. The gut microbiota is a control center for multiple aspects of your biology including your immune system, metabolism, and neurobiology. Many of the metabolic activities and developmental signals that the microbiota deploy are complementary to your own, affirming that humans are composite organisms consisting of microbial and human parts. The plasticity of the microbiota suggests that it may be a viable therapeutic target.