Via Brain Pickings‘ summary of Dr. Esther Sternberg’s The Balance Within: The Science Connecting Health and Emotions:
The same parts of the brain that control the stress response … play an important role in susceptibility and resistance to inflammatory diseases such as arthritis. And since it is these parts of the brain that also play a role in depression, we can begin to understand why it is that many patients with inflammatory diseases may also experience depression at different times in their lives… Rather than seeing the psyche as the source of such illnesses, we are discovering that while feelings don’t directly cause or cure disease, the biological mechanisms underlying them may cause or contribute to disease. Thus, many of the nerve pathways and molecules underlying both psychological responses and inflammatory disease are the same, making predisposition to one set of illnesses likely to go along with predisposition to the other. The questions need to be rephrased, therefore, to ask which of the many components that work together to create emotions also affect that other constellation of biological events, immune responses, which come together to fight or to cause disease. Rather than asking if depressing thoughts can cause an illness of the body, we need to ask what the molecules and nerve pathways are that cause depressing thoughts. And then we need to ask whether these affect the cells and molecules that cause disease.