Source:http://linkedlifedata.com/resource/umls/id/C0085256
MSH: In Chinese philosophy and religion, two principles, one negative, dark, and feminine (yin) and one positive, bright, and masculine (yang), from whose interaction all things are produced and all things are dissolved. As a concept the two polar elements referred originally to the shady and sunny sides of a valley or a hill but it developed into the relationship of any contrasting pair: those specified above (female-male, etc.) as well as cold-hot, wet-dry, weak-strong, etc. It is not a distinct system of thought by itself but permeates Chinese life and thought. A balance of yin and yang is essential to health. A deficiency of either principle can manifest as disease. (Encyclopedia Americana),NCI: In Chinese philosophy, yin and yang are opposite forces that form a whole. Everything contains both yin and yang in a balance that is always changing, such as hot and cold, day and night, and health and disease. In traditional Chinese medicine, disease is diagnosed and treated based on the balance of yin and yang.