External causing agents, as the Yang 陽 factor, including climatic, infectious and contagious.
Internal dysfunction, as Yin 陰 factor
Accidental and traumatic injuries, which are partly Yin and partly Ying.

Any disharmony of the equilibrium of Yin and Ying 陰 陽 or disturbance of their flow would causing weakness or illness. The task of Chinese doctor is to restore such imbalance of these two cosmological forces , (maintain the homeostasis in our modern term.) Treatment are not limited to herbal medicine, the also apply surgery, acupuncture, moxibustion, massage, breathing exercise, diet , and so on. The aim of the treatment is to enhancing or reducing the Yin or Ying in order to treat the body as a whole symptom, rather than any particular symptom.

The Chinese culture believes in the harmony of nature between heaven , earth  and Man. Man is composed just like any other beings or items in the world, by five xing  五 行 (Five elements), namely, Metal 金, Wood 木,  Water 水, Fire 火 and Earth 土. These five xing are in mutual production 相 生 and mutual conquest 相 克. Like fire conquers Metal, but can be controlled by Water.

Man constitutes a microcosm in the macrocosm in the universe, such a microcosm is balanced by two opposite cosmological forces of natural: Yin and Yang. Different parts of the body correspond to different Yin and Ying and the five xing.

Fire: heart , small intestine, tongue
Water: kidney and ears
Wood: gall-bladder and eyes
Metal: large intestine,
Earth: stomach, spleen and mouth

Important Chinese medical Literature:
Huangdineijing 黃 帝 內 經 ("Yellow Emperor's Manual of Corporeal Medicine"), by Huang Di 黃 帝 (?) (A legengary emperor). Including 2 treaties, namely Suwen 素 問 (questions and answers about living matter) and the Lingshu 靈 樞 (The vital Axis). Western Han ~200BC. Discussion of interrelationship between human and nature. Basic theories of pathology, diagnosis, prevemtion and treatment, and is consuidered as "bible " of TCM.

Shennong bencaojing 神 龍 本 草 經 (Pharmacopoeia of Heavenly Husbandman), by Shennong 神 龍 (?)(A legengary emperor). Explain the theory of Yin & Ying, drug interactions, indications, 365 items of medicine of mineral, plant & animal origin included in 3 categories according to their purposes. The 1st part are believed having the efficacy of nourishing or prolonging life; the 2nd part are generally non-toxic and are used to restore the constitution of the patient; the 3rd category are most toxic or having side effects, and are used for combating diseases. Western Han ~200BC.

Shanghanlun 傷 寒 論 (Treatise on Febrile Diseases) by Zhang Ji  張 机 (AD 150-219)[ Shanghan is refered as typhoid nowadays, but was refereed to "fever" in general]

Maijinf 脈 經 (Manual of the Pulses) by Wang Shuhe 王 叔 和 (AD 265-317). It described the techniques and theory of pulse taking.

Bencao gangmu 本 草 綱 目( The Great  Pharmacopoeia) by Li Shizhen 李 時 珍 (1518-1593) during the Ming dynasty. It describes plants, substance of animal origin, and minerals and metals together with their medical properties and applications. The text consists of over one million words and it describes in detail 1,800 kinds of medicinal plants, and 300 items had not been cited in previous works. It remains as the most influential and most  commonly referring  pharmacopoeia of TCM among Chinese.
Book of References:
Concepts of Chinese Science and Traditional Healing Arts: A Historical Review, by P Y Ho & F P Lisowski,World Scientific Publishing Co.1993, pp105.