It is believed that,in modern medical science,the epidemic diseases result from a variety of biological patho- gens or pathogens.In terms of traditional medical science,however,the epidemic diseases refer to illnesses that are fever- ish,prevailing,acutely infectious and extremely dangerous.In ancient literatures,it is generally defined as epidemic ill- nesses,diseases and plagues,etc.,the people in Song Dynasty also adopted the concepts.Under general condition,how- ever,the concept of pestilence is impossible to be separated from the epidemic diseases.Thus,all epidemic diseases and plagues are called pestilence.The epidemic diseases are very prevalent at southern areas of Yangtze River in Song Dynasty according to the literatures.Probably,it is related to the dotted lakes,warm and humid climates,dense population and frequent flow of people as well as the relatively more flourishing economy and chaos caused by wars,in which germs and virus are prone to reproduce.