I was at an MS research symposium a little over a year ago, and there was a doc there studying the EBV/MS linkage. What he said was that in the general population, a certain percentage of people have EBV antibodies detectable in their blood (I believe it was around 80%). In the population of people with MS, the EBV antibody detection rate was much higher (something like 96%). So his belief was that EBV plays some enabling role in MS, but may not be a "cause". His "best guess" was that EBV triggers the immune response that causes an MS flare.