The parasitic lifestyle of viruses makes them a challenging nemesis. Traditional lines of defense, such as antiviral drugs and vaccines, are difficult to develop, can produce undesirable side effects, and may lose efficacy if the virus mutates. Some scientists are now thinking outside the box, pointing out that we’re not alone in this fight. Trillions of microbes living in and on us—collectively known as the human microbiome—call the body home and need it to survive. Today researchers are testing whether they can draft these microbes into the immune system’s army to help fight viral invaders.

In the past few decades, scientists have learned a great deal about the gut microbiome, particularly the bacterial component. It has been