It is believed in modern medicine that epidemic consists of a group of diseases caused by varieties of biological pathogens or pathogeny for short. However, in terms of traditional medicine, epidemic refers to illnesses that are feverish, prevailing, acutely infectious and extremely dangerous. In ancient literatures, it is generally defined as popular illnesses, diseases and plagues, etc. and on the whole, people in Song Dynasty also adopted the concept. But in many cases, epidemic can not be accurately distinguished from popular diseases, therefore, they are generally called epidemic diseases or plagues. The south of Yangtze River in Song Dynasty was obviously more frequently stricken by epidemics than other regions according to the literatures, especially, the regions around the Taihu Lake. Probably, it is due to the dotted lakes, warm and humid climates, 〖JP2〗dense population and frequent flow of people as well as the comparatively more flourishing economy, in which germs and viruses are prone to reproduce. The high frequency of epidemics in those areas not only caused a great many deaths but also severely impacted the development of the local economy and social stability.