Is a diagnosis of ME/CFS (myalgic encephalomyelitis/chronic fatigue syndrome) always viral in origin, as distinct from CFS, which you have told us is viral 60% of the time?

It is difficult, given the wide array of interpretations in the use of such nomenclature, to be precise. Chronic fatigue syndrome is often a grab bag diagnosis as fatigue is such a common symptom of many maladies, so there are misdiagnoses and overdiagnosis at times which confound the picture. The description of this condition, as ME and CFS, is an accommodation to the historical development of two different labels, with chronic fatigue syndrome used in the United States and myalgic encephalomyelitis used in Europe, so common practice is to use both designations, ME/CFS. So it is not that the latter is a more definitive and accurately assigned label than just CFS, so we would simply leave things as we originally told you, that whatever you call it, ME/CFS is viral about 60% of the time, with respect to cases given that designation, and this will give an accurate picture given a high degree of false diagnosis.