Personally my guess on what ME/CFS is change throughout time depending on my personal bias of my own lived experiences and is even more so driven by the bias of which research has recently been published. The only thing I know for sure is that there isn't a simple explanation and studies looking to oversimplify things by reducing it to some phenonmenon we would have long since discovered are useless from the start.
Similarly the currently dominating hypotheses for Long Covid are:
- Viral Persistence
- Immune Dysregulation
- Latent virus reactivation
- Autoimmunity
- Dysfunctional Neurological Signaling and/or Neuroinflammation
- Coagulation problems
- Disruption of the Microbiome
I expect the list for ME/CFS would be extremely similar. Every item on the list itself contains various different theories. For example viral persistence can mean a viral reservoir or no reservoir but viral antigens. Furthermore different theories can be taking place in different people at the same time and can be heavily interacting with each other.
Today my personal belief is that the answer lies somewhere amongst autoimmunity, immune dysregulation and Dysfunctional Neurological Signaling and/or Neuroinflammation. If they once again find more and better evidence for viral persistence my beliefs are once again shifted into that direction.
Hypotheses are typically a reflection of the evidence and studies of that time. For example before the RCT with Rituximab CD-20 B-cell mediated autoimmunity was probably the most popular hypothesis. Now it is clear that if ME/CFS is an autoimmune disease that it won't just be driven by CD-20 B-cells and it also won't just be driven by those type of autoantibodies which have been searched for or looked at over and over again.
Most importantly Long Covid and to a slightly lesser degree, but still very much so ME/CFS, are extremely heterogeneous conditions. For Long Covid it is clear that there will be different driving causes in different people, I don't always expect ME/CFS to be one homogeneous condition with one root cause, I expect the answer will be far more complex and that there are different things that today are all considered to be ME/CFS. I think the complexity can be compared to MS and even though they know a million times more there, one still can't pinpoint one cause. This reflects the complexity of the disease and also the human body in which these theories are embedded in.
As a layman without any deeper biological, chemical or medical knowledge I can only asses the quality of current research on the basis of pretty basic faults or flaws in the methodology or reasoning. All to often these are unfortunately present.
I currently think we can get closer to an or one of the root causes, by doing well controlled studies of things that might only be happening downstream or are a reflection of certain symptoms. To me assessing mircocirulatory problems in vivo using modern measurement techniques seems like one of the most hopeful avenues to do this.