Over the years, attention has focused on two herpes viruses, Human Herpes Virus 6 (HHV-6) and Epstein-Barr virus (EBV). But there is no evidence that either virus causes Multiple Sclerosis, or MS.
One recent study, in the April 2010 issue of Neurology, casts doubt on the causative role of Epstein-Barr virus in MS, though there does seem to be an association. In the researcher's opinion, another virus might be responsible but not the Epstein-Barr virus. Another researcher disagrees.
You can read the Reuters report of this study here:
http://www.reuters.com/article/idUSTRE63E3XS20100415