@starlily88 stress likely does have some role to play in a lot of cases of illness.
Some more than others
It has apparently been found that cfs can be pretty reliably triggered by taking cortisol medications at the time of an infection
There are threads discussing it here on PR
Until the cause is found, we can’t dismiss any theory.
What are the causes for MS and lupus that you say have been found?
here is a quote of some of the discussion on PR about stress, cortisol and cfs below
Dr John Chia, who is known for his meticulous investigations into his ME/CFS patients' medical histories, discovered a fascinating association between corticosteroid prescriptions and ME/CFS. Dr Chia found that when patients came down with acute enterovirus infection, and were inadvertently mis-prescribed corticosteroids, this was almost a recipe for creating ME/CFS. Chia said he has literally hundreds of patients who developed ME/CFS as a result being given corticosteroids during acute viral infection. There is a thread about this
here.
Since stress can lead to elevated cortisol, such stress, in combination with an acute viral infection, could mimic exactly the corticosteroid mis-prescription circumstances Dr Chia describes.
It's hard to figure out exactly why
acute viral infection + corticosteroids = ME/CFS, but one simplistic explanation might be that the immunosuppression of corticosteroids allows the acute viral infection to penetrate into body tissue compartments it would not normally enter (like the brain), or to infect cell types it would not normally infect, and thereby allow the virus to insinuate itself more deeply into the body, making it harder to clear.