Potential common causes
One popular theory is that the spike protein causes both Long COVID and COVID vaccine injury. That doesn’t explain everything from before the pandemic such as ME/CFS and non-COVID vaccine injuries.
My theory is that the activation of the immune system is what opens the door to chronic illness. See this video. Vaccines must activate the immune system for lasting immunity to develop. Various people have theorized that infections lead to ME/CFS.
We should also be open to other theories. The three conditions seem to have elevated rates of autoimmune disease diagnoses (here’s data on LC and post vax).
What about PSSD?
See this post: