Psychosocial factors play an important though variable role in the predisposition, precipitation, and clinical course of most human illness. In a particular group of somatic diseases of uncertain cause—perhaps most appropriately classed as somatopsychic—the influence of psychosocial factors would appear to be greater than it is in diseases of simpler origin. Measles and essential hypertension, for example, could be one example of such a distinction. Central to the distinction is the degree to which the brain appraises or evaluates the situation, selects, and directs the response from the repertoire of the person challenged. Thus, whereas viral invasion may evoke little brain-directed response, situational or environmental strain may pose a challenge.
Threat, whether or not perceived consciously, calls forth, largely at the subcortical level, certain biological response patterns whose original adaptive purpose was to cope with the potential injury and to master the challenge. Such systems are already developed and operative
Kiely WF. Stress and Somatic Disease. JAMA. 1973;224(4):521-522. doi:10.1001/jama.1973.03220170047012