What is meant by pathogenesis? By pathbogenesis is meant the mechanism of the occurrence, developrment and outcome of diseases. The occurrence, development and outome of diseases are cosely related both to the body's vital qi and to the pature of the pathogenic factors. When the pathogens attack the human body, the body's vital qi is bound to rise against the pathogens, forming the conflict between vital qi and pathogens. Their conflict is bound to destroy the relative balance of yin and yang to cause the dysfunction of the zang-fu organs and meridians, or the disturbance of qi and blood. Thus bringing about a variety of local or general pathological changes. Notwithstanding various diseases and complicated clinical manifestations, their changes of pathogenesis, in general, are inseparable from the excess or deficiency between the vital-qi and the pathogens, the imbalance of yin and yang, or the abnormality of qi and blood and the dysfunction of the zang-fu organs and meridians. 

The conflict between the vital-qi and pathogens is not only related to the occurrence of a disease, but also directly affects its development and final outcome. Meanwhile, it has a direct influence on deficient or excessive changes of the syndromes. In a certain sense, the processes of many diseases are scen as those of the changes of excess and deficiency in the contest between the vital-qi and pathogens. Therefore, vicissitudes of vital qi and pathogens are considered to be one of the pathogenesises.

Basic Questions says "the exuberance of pathogens results in excess syndrome, while the depletion of essence-qi may bring on deficiency syndrome." Excess refers to excess of pathogens, or a pathogenic reaction with excessive pathogens as the principal aspect of the contradiction. That is to say, the pathogens are excessive, the vital qi is also strong. Clinically, a sort of pathological reactions of excess ensue with the fierce and obvious struggle between vital qi and pathogens, wbich is known as excess syndrome. It is mostly found in the early or middle stage of diseases caused either by the six exogenous pathogens or by the phlegm retention, undigested food, blood stasis, etc., characterized by high fever, mania, high voice and coarse breathing, abdoninal pain with tenderness, obstructive urination and defecation, and full and forceful pulse.

By deficiency syndrome is meant a syndrome caused by inadequste of vital qi, or a pathological reaction with the deficiency of vital qi as the dominant factor of the contradiction. That is, the vital qi become too deficient to resist pathogens because of the dysfunctions of qi, blood, body fluid, zang-fu organs and meridians. Consequently, the conflict of vital qi and pathogens may not produce intense pathological reaction and a series of insufficient manifestations arise, so called deficiency syndrome. This type of syndrome is mostly seen in patients with weak constitutions or in the later stage of a discase and in different kinds of chronic cases, marked by lassitude, pallid complexion, palpitation, short breath, spontaneous perspiration, night sweat feverish sensation in the chest, palms and soles or aversion to cold, cold extremities, feeble pulse, etc.

The rise and fall of vital qi and pathogens can bring about mixed syndromes of excess and deficiency in protracted and complicated diseases. Such syndrormes have mainly two kinds of pathological cbanges: deficiency syndromes mixed with excess ones and excess syndromes mixed with deficiency ones. However, under some particular circumstances, there wil be two types of pathological changes: true excess syndrome with pseudo-deficiency symptoms and true deficiency with pseudo-excess symptoms. The former is said to be "symptoms of pseudo-excess in extreme deficiency", while the latter is said to be "symptoms of pseudo-deficiency in extreme excess".From the above, to identify a deficiency or excess syndrome of pathogenesis we must see through the appearance to get at the essence and so and so only can we not be misled by false phenomena and accurately grasp the changes of excess or deficiency syndrome.

In the course of a discase, the conflict between vital qi and pathogens not only gives rise to their deficiency or excess, but also causes the outcome of a disease. When vital qi prevails over pathogens, the disease tends to improvement or even complete recovery. Otherwise, when pathogens prevail over vital gi, the disease tends to deterioration or even life cotmes to end.