The supposed connection between a period of stress before the onset of ME/CFS could be explained in a different biochemical way:
If you have the illness of
generalized anxiety disorder (a condition in which there is a lot continuous mental tension and/or worry, most likely caused by aberrant biochemistry in the brain), this anxiety disorder has been
shown to increase the risk of developing functional somatic disorders such as ME/CFS.
Now, as someone who experienced several years of generalized anxiety disorder (GAD) just before the virally-linked onset of my ME/CFS, I can tell you that once you have GAD, a lot of things in your life suddenly become stressful. Even things that you previously were able to do in a relaxed, easy, and enjoyable manner now become stressful, tense or worry-laden.
But this does not mean that your life itself has become more demanding or problematic (nothing externally had changed in my life at the time I developed GAD); it just means that your mental ability to cope with potential stressors has been greatly reduced, as a result of your generalized anxiety disorder. And it is important to distinguish between situations where your life itself has genuinely become more stressful, from situations where, because you developed GAD, your life
seems to be more stressful.
To give you an example: I had numerous friends at around the time that I first developed GAD; before GAD, visiting such friends was always a very enjoyable pleasure that I would look forward to; but as soon as I developed GAD, I started finding these very same social visits very stressful. Nothing had changed in the world or in my life, it was just the biochemistry of my brain that had changed, so that what was previously relaxed and enjoyable now became tense and fraught with anxiety.
So I suspect that when people report that they experienced a period of stress before the onset of ME/CFS, in many cases, this could be due to the fact that, unbeknown to them, they had developed anxiety disorder, which can then make it
feel as if life has become very stressful, when in fact nothing may have changed in your life.
You may
think that life has become more stressful, because you may be unaware that you have developed generalized anxiety disorder. This was my case: I did not at the time understand what had happened to my brain, because GAD can creep up on you slowly: it just seemed to me as if the
world and my life had got more and more stressful; it was only many years later, when I learnt about illnesses such as GAD, that realized it was not my external life that had become stressful, it was just my brain which was no longer able to cope with things, due to having GAD.
So I erroneously perceived the world and my life as becoming more stressful, when in fact the cause of this stress was internal to my brain, not external in the world.
So when ME/CFS patients say they experienced a period of stress before the onset of their ME/CFS, in reality, they may have just developed generalized anxiety disorder (which makes life feel more stressful), and this GAD condition is known to increase the risk of developing functional somatic disorders.
My GAD incidentally appeared as one of the symptom of my IBS, a bowel condition known to cause this anxiety disorder. This IBS itself I suspect was probably caused by contracting some type of pathogen, as my mother also developed IBS at exactly the same time, suggesting a common cause such as an infectious pathogen (post infectious IBS has been
linked to various pathogens including Campylobacter, Shigella, Salmonella and Escherichia coli).