Etiology is defined as various kinds of factors that cause diseases. As TCM sees it, etiology mainly includes the six exogenous pathogens, pestilence, the seven emotions, emotional frustration, improper diet, overwork, maladjustment between work and rest, traumatic injuries, insect or animal bites, phlegm retention, blood stasis, etc. Zhang Zhongjing of the Eastern Han Dynasty pointed out: "Despite nutmerous diseases, they would not exceed threc categories". Up to the Song Dynasty, Chen Wuze put forward the theory of the three categories of etiologic factors, i. e, exogenous pathogenic factors, endogenous pathogenic factors and non - endo - exogenous pathogenic factors. To be more specific, six pathogenic factors invading the body from the outside pertain to exogenous pathogenic factors; the seven emotions directly involving the internal organs are attributed to endogenous pathogenic factors; other pathogenic factors, such as improper diet, overwork, improper diet, traumatic injury, insect and animal bites, etc., are said to be non- endo - exogenous pathogenic factors. "The theory of the three categories of etiologic factors" caused strong reperçussions in later ages and promoted the research into the etiology.

In discerning etiology, besides having a sound knowledge of the objective conditions that possibly lead to illness, TCM centers attention on the clinical manifestations of diseases, and inquires into the etiology through analyzing the symptoms and signs so that a basis for treatment and medications may be is provided. This method is called "seeking cause of disease through differentiation of syndromes."

The six exogenous factors are a general tern for wind, cold, summer-heat, damp, dryness and fire. Under normal conditions, they are six climatic factors. TCM calls them the "six qi." They are not harmful to human beings but rather are the essential conditions of the growth of all living things in nature. Thus, the six qi will not cause diseases normally. However, when the six qi become too excessive or deficient, or when the body's resistance is too weak to adapt itself to the abnormal changes, the six qi will be changed into the pathogenic factors to attack the human body and cause diseases. They, as such, are known as "six exogenous pathogens" (liu yin). Of the six exogenous pathogens the five result fron the dysfunction of the zang fu organs rather than the exogenous factors invading the body from the outside though they also have the symptoms similar to wind, cold, dampness, dryness and fire. In order to distinguish them from the six exogenous pathogens, they are termed the "five endogenous pathogens", i. c., the endogenous wind, endogenous cold, endogenous dampness, endogenous dryness and endogenous fire.

The pathogenic features of the six exogenous pathbogens are as follows.

(1) The six exogenous pathogens have close relationships with the seasonal changes and living environment. For example,

diseases caused by wind-pathogen mostly occur in spring; sutnmer-heat diseases usually appear in summer; damp diseases often happen in later summer, dry diseases arise most often in autumn and cold diseases are often seen in winter. And the people who live in damp circumstances are susceptible to damp diseases, while the people living under a high tenperature are liable to contract fire or dryness diseases.