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The "Four Damages" (Si Sun) arise from human factors, whereas the "Four Deficiencies" (Si Bu Zu) stem from congenital endowment; the Four Damages occur during the acute phase of illness, while the Four Deficiencies are evident in one's baseline constitution. However, the Four Deficiencies may also originate from the Four Damages; one cannot assume that deficiencies do not exist outside the scope of damages. The Four Deficiencies refer to Qi, Blood, Yin, and Yang insufficiency.

Qi deficiency manifests as shortness of breath, difficulty in speech, and weak respiration. Even if the pathogenic factor is severe, distension or epigastric fullness (Pi Se) does not typically occur. In such cases, although dispersing and purging methods might seem indicated, the primary approach must be tonifying Qi.

Blood deficiency presents with sallow complexion and pale lips and mouth. Even with a severe pathogenic invasion, the face and eyes lack a yang-heat appearance. Although purgative and draining methods might appear suitable, the treatment must primarily focus on nourishing Blood.

Yang deficiency is characterized by cold extremities (Jue Ni), aversion to cold in the body, frequent diarrhea which worsens at night, or cold air exhaled from the mouth and nose. Despite severe pathogenic infection, there is typically no fever, prickly tongue coating, or dry thirst. For these syndromes, even if purgation, drainage, or clearing heat seems necessary, warming and tonification must be applied first. Once the deficiency recovers and the excess syndrome fully manifests, then treat the excess condition accordingly.

Yin deficiency naturally leads to the drying up of the five fluids and rough, scaly skin. Even with severe pathogenic infection, sweating may fail to occur when it should, or cold limbs may not appear when expected. In such cases, although purgation and drainage might be considered, nourishing Yin must be the initial step. Once Qi transformation recovers and body fluids return, the pathogen often retreats spontaneously. If the pathogen persists, clear and drain methods may be used judiciously. Premature use of purgatives and drainers will exacerbate the disease.

Regarding the above four deficiencies, combined with the previously mentioned four damages, the general principle is that one should not directly treat the pathogen using standard excess-clearing methods. The priority must be supporting the healthy Qi (Zheng Qi). Administer supportive medicines first; only when the excess symptoms fully manifest can one proceed to attack the pathogen. If attacking medicines cause the deficiency symptoms to reappear, one must revert to tonifying the deficiency. Support the healthy Qi to harmonize with the pathogen, and eliminate the pathogen to pacify the healthy Qi. These approaches should be adjusted and alternated until the pathogen is completely eliminated without damaging the healthy Qi; this constitutes skillful treatment.

The "Three Recurrences" (San Fu) refer to recurrence due to labor (Lao Fu), food (Shi Fu), and spontaneous recurrence (Zi Fu). Lao Fu occurs when a patient relapses after major illness due to overexertion. Even minor exertion, such as combing hair, washing, or bathing, can trigger recurrence. Upon recurrence, fever returns, and various symptoms re-emerge; the distinguishing feature is that the pulse is not deep and solid. Mild cases heal with rest; severe cases require strong tonification to regulate Ying and Wei (Defensive and Nutritive Qi) and harmonize the Zang-Fu organs. Recovery occurs only when interior and exterior are harmonized. Misuse of purgatives or cooling agents will lead to untreatable conditions; Anshen Yangxue Tang (Calming the Spirit and Nourishing Blood Decoction) is the principal formula.

If recurrence is caused by excessive food intake, the tongue coating will turn yellow again. Mild cases resolve by reducing food intake; severe cases require digestion-promoting and guiding-out methods. If recurrence happens spontaneously without obvious cause, it indicates that latent pathogens remain unresolved. One should inquire about the previous symptoms and the medications that resolved them, then reuse those same medications to cleanse the residual pathogens. Epidemic febrile diseases may recur two or three times. After multiple recurrences, symptoms of the Four Damages and Four Deficiencies often coexist. Treatment should follow the principles of adjustment and alternation described in the preceding section.