Infection with the Epstein–Barr virus or human herpesvirus 6 is associated with the initial onset of multiple sclerosis (MS), but new evidence suggests these viruses are not involved in MS relapse. Simpson and colleagues prospectively collected blood samples twice a year from 198 patients with MS. Epstein–Barr antibodies were detected in approximately 82% of patients, but the researchers found no relationship between viral load, disability progression or relapse rate.