Is it helpful to ask about 'percentages' here? Is oxygen 0% or 100% of the cause of you posting here? What about gravity? Whenever you have a causal question,

Is it helpful to ask about 'percentages' here? Is oxygen 0% or 100% of the cause of you posting here? What about gravity? Whenever you have a causal question, often taking a Pearlean perspective and asking about interventions would be more useful. For example, there is an EBV vaccine being worked on right now. If you believe OP that EBV infection is a necessary but not sufficient condition for 90% of MS cases, then it would be reasonable to say something like "if we intervened by making everyone got the EBV vaccine, then there would be ~90% fewer MS cases than in the counterfactual universe where everything else was the same but no one got the EBV vaccine." That sounds much more interesting and meaningful to me than talking about how "EBV is necessary but not sufficient".