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Stress (like mental, emotional, not just physical) causes the release of cortisol. Long term cortisol availability creates a tolerance, which then creates body-wide inflammation and a weakened immune system. I'll let you figure out what physical effects can result from that.
That's a single example.
OK, one example, of _one_ thing that can have a stress-related cause. But the issue is the tendency to ascribe almost _everything_ to psychological causes until proven otherwise (and it tends not to be proven otherwise precisely because once it's declared to be 'psychological' nobody bothers to look for physical causes). Just as once they would have said "God did it" or blamed it on the sufferer not accepting the teachings of Christ.
For Chronic Fatigue Syndrome, for example, sufferers had to go to court before it was admitted that the 'evidence' used by the NHS to declare it to be 'psychological' (basically due to the sufferers wrongly thinking they were ill and thus not making enough of an effort to pull themselves together, hence the treatment was to badger people with it into trying harder to be more active) was hopelessly flawed.
A much-touted study recommended therapy and gradually increasing exercise for patients with chronic fatigue syndrome. Problem is, it was based on bad science.
www.statnews.com
Most case histories I find of my own condition involve people being misdiagnosed with a 'psychological' condition, and then either falling down dead and getting a diagnosis post-mortem, or, if they are lucky, getting a brain scan and being diagnosed in time.
There is a direct line from the old days of declaring things like epilepsy or MS to be the work of Satan and his minions, and deciding things are due to the power of 'the mind'.
It also applies to the tendency to ascribe things that are really caused by social and economic problems to the 'bad thoughts' of individual sufferers. In both cases the mental health crowd are doing the same thing the priesthood traditionally did - diverting attention from the problems caused by people with power.