Viruses are ubiquitous members of every ecosystem, including the human body (1). A common misconception is that all viruses are harmful. In fact, their relationships to human hosts can be pathogenetic, symbiotic, or commensal. In the latter case, a virus benefits from but neither harms nor helps its host (2). Commensal viruses are as abundant as human cells, and their coexistence has persisted for millennia, permitting our immune systems to recognize them as part of us. Among the most abundant…