I'm quite sure its a combination of genes and infections (as EBV) during childhood and adolescense. And different combinations maybe cause the different kinds of MS (I think it's not only one disease anyway). They are so many hints that fit to this theory...
This is my opinion as a scientist/biologist and as a person concerned - my sister and me for example have obviously a similar genetic background, the same infections, e.g. EBV at the same age. We were both diagnosed within 6 weeks because we had our first relapse nearly simultaneously, had the same onset/kind of relapse, the same lesion load, the same number of new lesions in the control MRI, the same number of relapses, relapses usually at the same time. We have not lived together for 8 years, by the way. This is maybe an extreme case, and the studies with siblings with MS I've read are not consistent with "our" story, they show only similarities.
P.S. Sorry for my English, hope, it's not too difficult to understand.