This was originally posted in the Washington Post's "On Faith" section in response to their question: "Do you believe that faith can effect your health or is that a lot of new age nonsense?"
Faith is too vast a subject to generalize about-- its effects are indisputably not "New Age nonsense," unless you want to call Jesus nonsense. When he performed healings, he ascribed them to the faith of the one being healed, not to his own miraculous powers. Now that society has shifted its values radically toward materialism, faith healing is a suspect phenomenon. Proof requires an agent one can see -- such as neurotransmitters like dopamine, serotonin, and others -- to "explain" how the brain might trigger the immune system.
The minute you ask how mind influences body, you find yourself on the dividing line between secular and religious values. If only it were that simple. It tends to be true that people who hold deep religious beliefs (in the power of prayer, for example) are healthier than the norm, but the same holds true for vegetarians, rural dwellers, and anyone who is relatively free of anxiety, depression, and external stress. So if you are a vegetarian who lives on a farm in Iowa, no one can say that praying every day, once it is added to these other factors, significantly increases your well-being. (If you believe in a vindictive, angry God, it might even do the opposite.)
Well-being is subjective, and even though Western medicine largely discounts subjectivity, that in itself is a belief system. If you lose a thousand dollars in the stock market and it happens to be all the money you have, the effects on mind and body will be devastating compared to the same loss suffered by a billionaire. The difference comes down to a subjective feeling of helplessness and insecurity compared to a subjective feeling of security and abundance. Yet we know that throwing money at someone's unhappiness isn't a panacea. Rich persons don't necessarily feel secure -- depression and anxiety are tricky matters, often needing no external causes.
Nor is it easy even to define faith. Belief can be a form of denial, a defense that covers up underlying problems. Faith, as defined by Jesus, is the key to miracles and the Kingdom of God. This implies that faith creates transformation. It allows a person to transcend physical boundaries and step into the unknown (too bad that Lazarus isn't around to give his side of the issue). At the very least faith induces subjective well-being with about as much reliability as pharmaceuticals, minus damaging side effects. Which is not to imply that anyone should place absolute faith in faith.
My own view is that faith is a small part of the enormous field of consciousness. In our drugs-and-surgery society, we don't take enough advantage of non-material approaches. For example, studies in heart disease led by George Vaillant in the 1950s tried to explain premature heart attacks, then at epidemic levels. Vaillant found a mild correlation between artery disease and high cholesterol levels. This finding changed the course of treatment, ignoring the stronger correlation he found, which didn't fit the materialistic model. Men who faced their psychological problems in their twenties were considerably less likely to have a premature heart attack than men who didn't. And the evidence for the existence of a "cancer personality" has gained credibility through studies that show certain traits, such as suppression of emotion, raise the risk of many diseases (although a specific connection to cancer remains tenuous).
Although the deleterious effects of stress and its connection to the major epidemics of our time such as heart disease, cancer, and infection have been well-documented over several decades, there has been little research on what peace of mind does to our biology. I can see how a sense of "peace that passes understanding," and an internal state of euphoria, could lead to healing. There is more and more evidence that when the mind is at peace and also happy, there are biological consequences. An internal state such as this is more than merely subjective well-being. Neurotransmitters such as dopamine, oxytocin, opiates, and serotonin are simultaneously secreted. These neurotransmitters are associated with euphoria, self-confidence, and a pleasurable feeling. They also happen to be immunomodulators, in that they fine tune the activity of the immune system and are associated with a return to homeostasis (homeostasis is a state of dynamic non-change in our biology in the midst of a changing environment, and disease is a disruption of homeostasis). It is, therefore, becoming increasingly clear that the mind and body are inseparably one and that which we call faith is an important component in the phenomenon of healing. It is no accident that the word health, the word healing, and the word holy, all imply a return to the memory of wholeness.