In ancient China the disease were classified according to the different causes. In NEIJING the causes of the diseases (Xie – Evils) are given as following:
The cause of the Evils is Yang or Yin. Those which are Yang are Wind, Rain, Cold and Heat; those which are Yin are: food, sexual intercourse, the disruption of balance between Yin and Yang and emotions.
But no matter whether the disease of an organ is due to internal or external cause, the reason of its breaking out rests in the fact that there is something wrong with the Qi of the respective organ. The theory concerning the causes of the diseases was further developed by Zhang Zhongjing from the later Han dynasty (25-220 A.D.) who classified the diseases as arising from 1. external causes 2. internal causes and 3. sexual intercourse and wounds as resulting from weapons, worms and reptiles. During the Sung dynasty (960-1273 A.D.), Chen Wuze advanced another theory on the classification of diseases according to their cause which practically has been existing till now.
External causes (waiyin) – the Six Qi (Liu Qi), sometimes called the Six Evils (Liu Xie) or the Six Excesses (Liu Yin). These are: Wind (Feng) corresponding to spring, Cold (Han), corresponding to winter, heat (Shu) corresponding to summer, Damp (Shi) corresponding to mid-summer, Dryness (Zao) corresponding to autumn and Fire (Huo) corresponding to all of the five seasons mentioned above. Under normal conditions they are called the Six Airs (Liu Qi) and they manifest the seasonal climate and are helpful to the development of the beings, but under abnormal conditions they are called the Six Evils or the Six Excessives and are noxious. The diseases are closely connected with the seasons. Thus in spring mostly diseases due to the wind Evil occur, in late summer diseases due to the Damp Evil etc. Sometimes the body can be invaded by several Evils simultaneously, or it can happen that the Evil after having invaded the body does not manifest itself immediately but after some time. This is the so called Dormant Qi (Fu Qi). In NEIJING we read the following:
It is said: When people are injured through the severe cold of Winter, the sickness will recur in Spring. When people are hurt through the wind in Spring they will not be able to retain their food in Summer. When people are hurt through the extreme heat of Summer, they will get intermittent fever in Fall. When people are hurt through the humidity of Fall, they will get a cough in Winter.
Infectious diseases (yili) are also listed among the diseases arising from the external causes. They are due to the abnormal Qi between Heaven and Earth and occur when there are abnormal changes of the climate (cold summer, long lasting dryness etc.) or because of lack of hygiene.
Internal causes (neiyin) – the Seven Emotions (Qi Qing) which comprise joy (xi), anger (nu), grief (you), strain due to excessive mental activity (si), sadness (bei), fear (kong), anxiety (jing). These emotions are associated with the Five Zang Organs. Thus anxiety and joy belongs to the heart, anger to liver, strain due excessive mental activity to spleen, sadness and grief to lungs, fear to kidney. Normal alternation of these Seven Emotions keeps the Five Zang Organs in balance, but if one emotions suppresses the other or is suppressed, a disease appears. The seven emotions exert influence upon the organs and the diseased organ can bring about corresponding emotions. Thus anxiety and joy destroy the heart and a patient suffering from heart disease is afflicted by anxiety.
Other causes (bu nei bu wai) which are neither of the external nor the internal origin. The following are listed: excessive eating and drinking (yinshi), exhaustion (laojuan), excessive sexual intercourse (fangshi bu jie), wounds caused by weapons (chuangshang) and injuries caused by insects, reptiles and animals (chongshou-shanghai).