Disease means the damage of health. TCM believes that the healthy state implies the balance of yin and yang, the manifestation of which is high coordination and unity between the viscera, the meridians, qi, blood and body fluid as well as the body and the external environment. If such a state of high coordination and unity is broken by various pathogenic factors, disease will be caused. The occurrence of disease is actually concerned with pathogenic factors and healthy qi (body resistance). Besides, constitution is also an important factor responsible for the occurrence of disease.