EBV has been known to mess up B cells to a different state for at least forty years. Once you have had an EBV infection your B cells are never the same again. But the problem is that this happens to more or less all of us (95%+ I think) so EBV messes up your B cells to a state that everyone else has and is completely normal for modern human beings. So this messing up cannot really be 'the cause' of anything. The more subtle issue is whether or not certain diseases specifically require EBV to have been around. That we know to be the case. Certain forms of Burkitt lymphoma and nasopharyngeal carcinoma need EBV. Lymphomas can be cured by removing EBV. The same sort of thing goes for Helicobacter, which most of us have. Certain lymphomas disappear if you remove the Helicobacter. I think a lot of people working in this field think that the presence of EBV may 'permit' certain types of ME from occurring, but something else must explain why some people have this sort of ME and most people do not.