凡人大劳、大欲、及大病、久病后，气血两虚，阴阳并竭，名为四损。当此之际
，忽又加疫，邪气虽轻，并为难治，以正气先亏，邪气自陷，故谚有云∶伤寒偏死下虚人，
正谓此也。
盖正气不胜者，气不足以息，言不足以听，或欲言而不能，感邪虽重，反无胀满
痞塞之证，误用承气，不剧即死。以正气愈损，邪气愈伏也。
若真血不足者，面色萎黄，唇口刮白，或因吐血崩漏，或因产后亡血过多，或因肠风脏毒所
致，感邪虽重，面目反无阳色，误用承气速死，以营血愈消，邪气益加沉匿也。
若真阳不足者，或四肢厥逆，或下利清谷，肌体恶寒，恒多泄泻，至夜益甚，或口鼻冷气，
感邪虽重，反无发热燥渴苔刺等证，误用承气，阳气愈消，阴凝不化，邪气留而不行，轻则
渐加
萎顿，重则下咽立毙，若真阴不足者，自然五液干枯，肌肤甲错，感邪虽重，应汗无汗，应
厥不厥，误用承气，病益加重，以津液枯涸，邪气涩滞，无能输泄也。凡遇此等，不可以常
法正治，当从
其损而调之，调之不愈者，稍以常法治之，治之不及者，损之至也。是故一损二损，轻者或
可挽回，重者治之无益，乃至三损四损，虽卢、扁亦无所施矣，更以老少参之；少年遇损，或可调治；老年遇损，多见
治之不及者，以枯魄独存，化源已绝，不
复滋生也。
凡是人在过度劳累、纵欲过度，以及大病、久病之后，气血两虚，阴阳俱竭，这种情况被称为“四损”。在这个时候，如果忽然又感染了疫病，虽然邪气不重，但也难以治疗，因为正气先已亏损，邪气自然内陷，所以谚语说：“伤寒偏死下虚之人”，正是说的这个意思。
如果是正气不胜任的情况，表现为气息微弱不足以喘息，声音低微不足以听清，或者想说话却说不出来，即使感受的邪气很重，反而没有腹胀满、痞塞的症状，如果误用承气汤（攻下实热的方剂），病情不加重就会死亡。这是因为正气更加受损，邪气更加潜伏隐蔽。
如果是真血不足的情况，面色萎黄，嘴唇和口角苍白，可能是因为吐血或崩漏（非经期大量阴道出血），也可能是因为产后失血过多，或者是由于肠风（肠道风热便血）和脏毒（湿热毒邪蕴结肠道所致便血）所导致，即使感受的邪气很重，面目反而没有阳热之色，如果误用承气汤，会迅速致死，因为营血进一步消耗，邪气更加深沉隐匿。
如果是真阳不足的情况，或者四肢厥冷逆乱，或者大便泻出未消化的食物，身体怕冷，经常腹泻，到了夜晚症状更加严重，或者口鼻呼出冷气，即使感受的邪气很重，反而没有发热、烦躁口渴、舌苔生刺等症状，如果误用承气汤，阳气进一步消亡，阴寒凝滞不能化解，邪气停留而不能排出，轻则逐渐萎靡不振，重则药咽下立即毙命。
如果是真阴不足的情况，自然会出现五液（汗、涕、泪、唾、涎）干枯，肌肤粗糙如鳞甲交错，即使感受的邪气很重，应该出汗却无汗，应该出现四肢厥冷却不厥冷，如果误用承气汤，病情会更加加重，因为津液枯涸，邪气涩滞，无法通过输布排泄出去。
凡是遇到这些情况，不可以用常规的正治法（针对实证的热者寒之、寒者热之等直接治疗方法），应当顺着其虚损的情况进行调理，调理后如果不痊愈，再稍微用常规方法治疗，如果治疗还来不及见效，说明损伤已经到了极点。因此，一损、二损，轻症或许还可以挽回，重症治疗也无益，至于三损、四损，即使是扁鹊这样的神医也无法施展手脚了。还要结合年龄来参详：少年遇到虚损，或许可以调治；老年遇到虚损，多见治疗不及的情况，因为枯槁的魂魄独存，化源已经断绝，不再能滋生生机。
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.