Surprisingly the system belongs to the mind. If the mind is pleased with some idea(good or bad), the system plays along with the latter, to an extent that it can be quite disastrous.
I remember reading this particular short story(Barin Bhowmik's ailment) by Satyajit Ray. The story revolves around this person. He experiences a slight pain in this back(i'm not sure). His mind develops the small pain to something really big. Finally, the protagonist comes to the conclusion that his death is not far off. Though, he is not afflicted with any ailment. It's his mind that's wreaking havoc. A concerted response from all the body systems.
Prepostorous, yet realistic.
The same happens in our environment too. Of course without our knowledge. Or because we fail to acknowledge it.
A similar event happened at home sometime back.
Mother was of the opinion that she was weak. Well she is a little anaemic. Apart from that, she is hale and hearty. This particular thought found ground in her. Father and i had to eliminate this 'pest' from her head. We used all tactics possible, spoke to her in a polite manner... asked to her eat a little more.....yelled at her(one can obviously guess the person behind this)... requested her to reduce the work at home... to no avail.
Later, mother was taken to a doctor. The doctor drove home the point that it was the thought that was eating her body than any physical illness.
Eventually, mother did come out of her belief.
Thank heavens!! Were we relieved!!