Pathogenic Factors
These are divided into three main groups, exogenous pathogens, mental pathogens and various miscellaneous pathogens. 'Phlegm and humour' and 'stagnant blood' are pathological products; once they are formed new pathological changes will ensue so they are considered to be secondary pathogens.

Pathological factors serve as a generalization of clinical symptoms and signs, reflecting the struggle of normal qi and pathogenic qi. By differentiating the clinical symptoms and signs the cause of the disease can be traced, and then treatment can be determined. In order to do this the diseased organs must be defined and the pathogen causing that disease must also be diagnosed. This is called the 'determination of treatment on the basis of the differentiation of a syndrome', and it is the basis of diagnosis and treatment in Chinese medicine.

The Exogenous Pathogens
These refer to six relatively abnormal meteorological conditions; wind, cold, summer heat, damp, dryness and heat (fire, warmth). The diseases caused by these pathogens include most viral, bacterial and protozoal diseases and some 'allergic' conditions such as urticaria.

	cold and damp				normal qi of
	invade pi spleen			pi-spleen is
	impairing its function		        overpowered

	
				
	symptoms of disease—			impairment of the
	anorexia. abdominal distention,		function of pi-spleen
	pain, diarrhoca, cold extremities,
	greasy white tongue, deep thready  pulse

Wind
This pathogen is characterized by movability (of symptoms) and changeability. The clinical manifestations are abnormal limb motion, such as spasm or twitching, and a wandering symptomatic site as in urticaria or arthralgia. The symptoms may vary in intensity and they usually include a dislike of wind, fever, sweating, headache and an itchy throat.

Cold
Invasion of cold will consume the yang causing a contraction of the channels and the blood vessels, and therefore a poor circulation of qi and blood. The symptoms are those of a slight fever, a dislike of cold, hypohydrosis, headache, muscular pain and spasm, and occasionally a dark blue and painful area in the local muscles and skin; a frozen shoulder is a good example of the pathogen cold.

Summer heat
This only occurs in the summer; it damages the yin and may progress to affect the level of consciousness. The symptoms are excessive body heat, profuse sweating, thirst, a dry mouth, dry red skin and, in severe cases, delirium (this is known as heat exhaustion in Chinese medicine). Summer heat may combine with wind and cause convulsions. Summer heat often combines with damp to produce dizziness, nausea, a stuffy sensation in the chest and general malaise.

Damp
Diseases caused by damp are sticky, muddy, greasy and stagnant. Damp causes a generalized heavy feeling associated with distension, dizziness and a heavy head, general malaise and a stuffy sensation in the chest. The patient may also complain of abdominal swelling and an exudative and prolonged skin disease.

Dryness
Dryness consumes yin fluid. There may be a dry sore feeling in the nose, mouth and throat, a coarseness of the skin, or a cough with scanty sputum and possibly haemoptysis. Tuberculosis is an example of the pathogen dryness.