No one knows the cause yet, which is why studies like this are important.
Please don't idly suggest that CFS is just something psychosomatic or behavioral. The disease is much more debilitating than that, and this very common psychological misdiagnosis most often leads to dangerous advice. For example, CFS patients are often told by doctors to excercise, which actually only makes the symptoms even worse.
I have severe CFS, but I personally don't take it badly when someone suggests it's psychosomatic. Since everything I know and everything I feel is a hallucination of the mind, I have no way of knowing what my subconsciousness is influencing. "Psychosomatic" also doesn't mean "it's your imagination", but that it may be an interplay between mind and body. The mind does control many responses and hormonal activity.
I think that this suggestion that CFS might be caused by bad diet is even more irresponsible than telling it is psychological. CFS patients would wish that it were that simple.
CFS onset is often abrupt, and can often happen to otherwise very healthy and active people. For example, a common story is that the patient becomes sick from a viral infection and then gets stuck with chronic fatigue syndrome even after the infection goes away.
If we can determine for sure that it's happening a at a much higher rate than in the past, we have a pretty good idea that the cause is the conditions that we live in. Diet that is one thing in which the changes over the years have been similar across different regions. If it was air pollution, the increased rates would show up in some regions but not others. Ditto radiation.
I don't think this is like autism in that we were mostly oblivious to it in the past, but I could be wrong. Enlighten me.
Even today it is very hard to get an accurate CFS dianosis. Patients often are diagnosed with as suffering from a viral infection, or psychological, neurological, or immune disease. They almost always only get a CFS diagnosis after all the other treatments fail. I definitely would not be surprised if it turned out to be present in the past, but misdiagnosed.
One thing that certainly does not help is that the primary symptom of ME/CFS, post-exertional malaise, is counter intuitive. The pain and fatigue comes after a delay, which hides the cause and effect. Everyday physical activity can also be enough to bring back symptoms. This means that if you don't know what you are looking for then ME/CFS will appear to come "out of nowhere", like a mystery chronic illness.
I don't think severe limitations in physical mobility were as common in the past. I'm not the only one who thinks this. The creators of WALL•E clearly think it as well.
Please don't idly suggest that CFS is just something psychosomatic or behavioral. The disease is much more debilitating than that, and this very common psychological misdiagnosis most often leads to dangerous advice. For example, CFS patients are often told by doctors to excercise, which actually only makes the symptoms even worse.