What does science actually say about psychological issues manifesting in severe physical illness?
Physical illnesses like multiple sclerosis, Parkinson's, irritable bowel syndrome, Crohn's, etc often lead to mental symptoms like depression and anxiety, even psychosis, there is no controversy about that.
The controversy comes from the reverse: the idea that mental symptoms (like anxiety or feeling stressed) can transform into physical illnesses. That notion is known as a psychosomatic illness, or a somatoform disorder.
There is a subtle difference between psychosomatic and somatoform disorders:
Psychosomatic illness = has actual objectively-measurable physiological symptoms (such as blood pressure) that are affected and aggravated by psychological factors such as emotional state.
Somatoform disorder = subjective feelings of illness that are untestable by medical tests.
The highly dubious and slightly crazy psychiatrists like Simon Wessely who entered ME/CFS research in the 1980s and recast ME/CFS as a a psychologically-caused disease considered ME/CFS to be a somatoform disorder, because there are no medical tests which can find any substantial pathology in ME/CFS.
So the idea is that your mental state or thoughts can cause an actual illness. There's almost no evidence for that, apart from obvious simple things like stress can raise blood pressure, which is true.
Wessely stated that ME/CFS was simply caused by a belief: he said that patients believed they were ill, and then this thought in their mind actually transformed into the illness of ME/CFS. See Wessely quotes
here.
Patients then started being blamed for creating their own illness just by having the wrong thoughts in their mind, and treatment of ME/CFS patients turned into a form of exorcism of the thoughts that these mad psychiatrists said were causing ME/CFS.
Treatment of young teenage ME/CFS patients in the UK is still in effect based on exorcism. Not just exorcism of the patient's thoughts, but exorcism of the parent's thoughts as well: if parents actually think that there child is really ill, that is deemed to be supporting the the belief system of the patient, and these parents are then reported to social services for child abuse.
These crazy psychiatrists think the parents are encouraging the child's "illness beliefs" and thereby causing the ME/CFS.
I know that sounds unbelievable, but reporting parents for child abuse happens regularly with teenage ME/CFS cases, as Jane Colby of the Tymes Trust Children's ME Charity will tell you.
At timecode 25:37 of episode 4 of
this set of podcasts, Jane Colby states that in recent years she has dealt with over 200 cases of parents being reported for the "child abuse" of believing that their kid's illness is real
Note that there are two podcasts on the page named "episode 4", the Colby one is the second of the two.