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In general, after excessive labor, excessive desires, or following severe and chronic illnesses, when both Qi (vital energy) and Blood are deficient, and Yin and Yang are exhausted, this condition is termed "Four Damages" (Si Sun). At such a time, if an epidemic pathogen is suddenly contracted, even if the pathogenic factor is mild, it remains difficult to treat because the healthy Qi has previously declined, allowing the pathogen to sink inward. Hence, the proverb states: "Typhoid fever tends to kill those with deficiency in the lower body," which precisely refers to this situation.

When Healthy Qi is insufficient to withstand the pathogen, symptoms include weak breathing that is insufficient for respiration, a voice so faint it cannot be heard clearly, or the desire to speak but inability to do so. Even if the contracted pathogen is heavy, there may paradoxically be no signs of abdominal distension, fullness, or epigastric obstruction. If Chengqi Tang (Cathartic Decoction) is mistakenly used, the condition will either worsen severely or lead to death. This occurs because Healthy Qi is further depleted, and the pathogen becomes more deeply entrenched.

If True Blood is insufficient, the complexion appears sallow and yellow, and the lips and mouth look pale. This may result from hematemesis (vomiting blood) or metrorrhagia (profuse uterine bleeding), excessive blood loss postpartum, or conditions caused by Changfeng (intestinal wind causing bloody stool) and Zangdu (visceral toxin causing bloody stool). Even if the contracted pathogen is heavy, the face and eyes lack yang-heat coloration. Mistaken use of Chengqi Tang leads to rapid death, as Ying-Blood (nutritive blood) is further consumed, and the pathogen becomes increasingly deep-seated and hidden.

If True Yang is insufficient, symptoms may include cold extremities (Jue Ni) or diarrhea with undigested food. The body feels aversion to cold, frequent diarrhea is common, worsening at night. There may be cold air exhaled from the mouth and nose. Even with a heavy pathogen, there is typically no fever, restlessness, thirst, or prickly tongue coating. Mistaken use of Chengqi Tang causes further depletion of Yang Qi, leading to stagnation of Yin-cold without transformation. The pathogen remains stagnant and immobile; mildly, the patient gradually becomes listless and debilitated; severely, death occurs immediately upon swallowing the medicine.

If True Yin is insufficient, the Five Fluids (sweat, nasal mucus, tears, saliva, and saliva) naturally dry up, and the skin becomes rough and scaly like fish scales. Even if the contracted pathogen is heavy, sweating should occur but does not; Jue Ni should occur but does not. Mistaken use of Chengqi Tang aggravates the disease because body fluids are exhausted, causing the pathogen to become stagnant and unable to be transported and excreted.

For all such cases, one must not apply standard direct treatment methods (Zheng Zhi). Instead, regulation should follow the nature of the damage. If regulation fails to cure, then slight application of standard treatments may be attempted. If treatment still fails to take effect, it indicates the damage has reached its limit. Therefore, for First and Second Damages, mild cases might be reversible, while severe cases yield no benefit from treatment. For Third and Fourth Damages, even masters like Lu (Qing) and Bian (Que) would find their skills useless. Age must also be considered: young people encountering damage may potentially be regulated and treated; however, elderly people encountering damage often prove untreatable because their withered Po (corporeal soul) remains alone, the source of transformation is exhausted, and regeneration can no longer occur.