[The development of the theory of "San Xu Zhi Yi" in the Song Dynasty].

The theory "San Xu Zhi Yi" means epidemic diseases are mainly caused by changes in nature, deficiency of human energy and etiology (irregular sources of diseases). It was developed by some physicians in the Song Dynasty on the basis of <i>Su Wen Yi Pian</i> and <i>Shang Han Li</i>, with its relevant prevention and treatment. It was systematically discussed by some physicians, such as Han Zhihe, Pang Anshi, Zhu Gong, Guo Yong and Chen Wuze, in terms of spatio-temporal factors, the effects from weather, geography, human constitution and the seasonal principle of epidemic diseases towards its occurrence and prevalence. It was believed that the irregular sources of diseases (Xie) involve coldness and abnormal Qi. It was also believed that diagnoses needed to pay sufficient attention to environment, season, region, and the development of the diseases. Its prescription should involve gypsum and folium for clearing away heat and toxic materials, protecting the stomach, and adjusting diet and lifestyle. This theory and method of dealing with epidemic diseases in the Song Dynasty had great influence on the school of epidemic febrile disease, the school of warm-heat and the school of latent qi.