In our bodies, microbial genes outnumber human genes by about 100 to 1. The human microbiome (the bacteria, fungi, and viruses that live in us) has been linked to gastrointestinal diseases, immune functioning, type 2 diabetes, obesity, and more recently cognitive and psychiatric functioning. Today we speak with Tanya Nguyen, Ph.D., a neuropsychologist whose research is helping to clarify this link.