Thriving alongside trillions of bacteria in the human digestive tract is a vast population of viruses. Ever since one such denizen of the gut microbiome, known as picobirnavirus, was first spotted decades ago, researchers thought it directly infected human tissue and played a role in gastrointestinal illness. But researchers at Washington University School of Medicine in St. Louis and collaborators have just rewritten that story: This widespread virus infects gut bacteria, not human cells.