According to Sheila Neal there wasn't overwhelming support for a physiological explanation at the time. My guess is that as time went on (and especially as we went into the sixties) a psychological explanation became more fashionable.@AR68 I thought you might like this paper from the BMJ Epidemic Myalgie Encephalomyelitis (nih.gov) Not for the text which seems a fairly routine 1957 description, but what are we to make of the juxtaposition? The continuation with "subliminal advertising" seems, at best, unfortunate.
Do they think, as some others do, that suggestion and suggestibility plays a part?