New treatments for multiple sclerosis (MS) focused on B cells have created an atmosphere of excitement in the MS community. B cells are now known to play a major role in disease, demonstrated by the highly impactful effect of a B cell-depleting antibody on controlling MS. The idea that a virus may play a role in the development of MS has a long history and is supported mostly by studies demonstrating a link between B cell-tropic Epstein–Barr virus (EBV) and disease onset. Efforts to develop antiviral strategies for treating MS are underway. Although gaps remain in our understanding of the etiology of MS, the role, if any, of viruses in propagating pathogenic immune responses deserves attention. Clinical studies show that depletion of B cells reduces disease burden in both relapsing-remitting and progressive multiple sclerosis (MS) patients.B cell-tropic viruses may trigger aberrant immune responses in MS in genetically susceptible individuals owing, in part, to a failure in viral surveillance and clearance.The most compelling data supporting an etiologic role for viral involvement in MS have emerged for Epstein–Barr virus (EBV).Targeting mechanisms by which EBV is thought to participate in MS pathogenesis provides an opportunity for new drug development in MS.