Some people ask me for "proof" when I tell them that I used the Law of Attraction and the mind/body connection to heal myself of chronic pain and several chronic illnesses. Beyond the fact that I now FEEL great, have no more symptoms and have not touched a pain pill in a year there is also a science that is the study of JUST how strong the mind/body connection is. The medical study of how behavior and thoughts affect you physically is called Psychoneuroimmunology (PNI). There are numerous studies that you can find online that have "proof" and it is also interesting to read about how far psychoneuroimmunology has come. I believe the study of PNI will help medicine become more integrative and complementary as time moves forward.
The definition is as follows:
Psychoneuroimmunology (PNI) investigates the relations between the psychophysiological and immunophysiological dimensions of living beings. PNI brings together researchers in a number of scientific and medical disciplines, including psychology, neuroscience, immunology, physiology, pharmacology, psychiatry, behavioral medicine, infectious diseases, and rheumatology. The profound interest of PNI is in interactions between the nervous and immune systems, and the relation between behavior and health. Despite the protean approach to research, the outcome common to all research endeavors is the discovery of new information, or novel evidence, which contributes to the continuing and cumulative generation of knowledge. It deals with, among other things, the physiological functioning of the neuroimmune system in states of both health and disease; malfunctions of the neuroimmune system in disorders (autoimmune diseases, hypersensitivities, immune deficiency), the physical, chemical and physiological characteristics of the components of the neuroimmune system in vitro, in situ, and in vivo. PNI also involves endocrinology and is sometimes referred as psychoendoneuroimmunology (PENI).
The "Bible on PNI" to purchase is "Psychoneuroimmunology, Two-Volume Set, Volume 1-2, Fourth Edition" by Dr. Robert Ader -- Dr. Ader has been called the "father of pyschoneuroimmunology".