"Note, though, that EBV would then be in the "necessary but not sufficient" category. There's something about the interaction of particular human immune systems with EBV infection that pushes things over into the pathological state of multiple sclerosis, and we don't really know how to identify these people. But that fits with what we know about infectious disease in general - everyone's different. The situation with Guillian-Barré is similar - a small number of people tip over into neurological pathology, for reasons unknown, and that one also often seems to follow some sort of viral infection."