!@#$%. The forum software lost my post, which I find disappointing, but I hope I can remember what I wrote:
We are psychosocial animals and it is natural for people to be influenced by others; social trends, fashion, fads, peer pressure, gossip, hype, role models, cue taking, etc. We have so-called mirror neurons which help us to mimic or learn from the behaviour/actions of others. Dynamics can occur in numbers, eg people being crushed in stampedes, tension in the atmosphere followed by an eruption of brawling which waves across a drunken crowd, etc). Mass panic occurs in response to a public threat, but may be enhanced by responding to cues from how others are reacting to the threat.
Of course, influence is generally mild and subtle compared to "hysteria", but any phenomenon usually has outliers or extreme examples too. So, although it is a subject I have yet to explore in much detail, I tend to believe it is possible that mass psychogenic illness can occur in extreme scenarios, and that potential mechanisms exist. However, it is sloppily overused as an explanation and should not be confused with mass panic (but seems related to anxiety and cue taking).
It still makes me chuckle when I hear about how in the Victorian era, it seemed to be relatively common practice for bored or frustrated housewives to be diagnosed with "hysteria" and then be treated with prolonged sexual stimulation by a physician. Vibrators were invented to reduce the workload. IIRC, when Charcot (famous neurologist and supposed leading expert on hysteria at the time) gave private demonstrations of "hysteria", there were the usual epileptics and other such patients, but some were demonstrations of what happens when he sexually stimulated a woman to climax. This is how out of touch these people were about certain human functions. Keep in mind that not only was Freud one of Charcot's students, but much of society's perceptions about psychosomatic illness filtered down from this ridiculous era in medicine.
ME has been defined for several decades and CFS was defined in the late 80's. Many of the leading psychobabblers spent their training and formative years during these times, and much of the psychobabbling about ME/CFS has been directly imported as pre-existing concepts from psychology and psychiatry about psychosomatic illness, somatization, conversion disorder, medically unexplained functional illness, etc. These concepts have barely evolved much past the Victorian era, but have been rehashed to adapt to updates and criticisms without a fundamental rethink of the underlying assumptions. Much of it appears to be highly questionable and has not been subjected to enough scientific skepticism.