A connection between the human herpesvirus Epstein-Barr and multiple sclerosis (MS) has long been suspected but has been difficult to prove. Epstein-Barr virus (EBV) is the primary cause of mononucleosis and is so common that 95 percent of adults carry it. Unlike Epstein-Barr, MS, a devastating demyelinating disease of the central nervous system, is relatively rare.