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After four or five days, or even ten-plus days of contracting a cold injury (Shanghan), if there are neither exterior syndromes nor interior syndromes, Xiao Chai Hu Tang (Minor Bupleurum Decoction) should be used universally, with modifications based on specific symptoms.

Yang syndromes refer to the Taiyang (Greater Yang), Yangming (Bright Yang), and Shaoyang (Lesser Yang) meridians. Initially, there is fever, headache, and stiffness in the neck. Later, lips become dry and cracked, mouth and throat feel parched, restlessness and thirst occur with a preference for cold drinks, the facial complexion is lustrous and bright, speech is clear and loud, hands and feet are warm, nail beds are ruddy, stool may be constipated or hard, urine may be red or scanty, and the pulse is floating, surging, rapid, and large. These are manifestations of Yang syndromes. Treatment may involve sweating (diaphoresis), harmonizing, or purging methods.

Yin syndromes refer to cold syndromes where pathogens directly invade the Taiyin (Great Yin), Shaoyin (Lesser Yin), and Jueyin (Terminal Yin) meridians. Initially, there is no headache, no body heat, and no thirst; instead, there is aversion to cold or shivering. The face appears pale as if scraped by a knife, the body feels heavy and difficult to turn, vomiting and diarrhea occur, urine is clear or white, the patient curls up wanting to sleep, hands and feet are cold, nails appear cyanotic, the facial complexion is dull and lifeless, and the pulse is deep, choppy, fine, and small. These are all manifestations of Yin syndromes. Treatment may involve warming or tonifying methods.

Yang toxicity (Yang Du) results from excessive Yang qi and sudden exhaustion of Yin qi. It may arise from a Yang syndrome mistakenly treated with warm-hot herbs, or transforming into Yang toxicity after emesis or purgation. The patient suffers from high fever and agitation, may run around wildly cursing, see hallucinations of ghosts, vomit pus and blood accompanied by red rashes, have a curled and scorched-black tongue, a nose resembling soot, swollen and painful throat, yellow-red diarrhea, and six pulses (radial arteries at both wrists) that are surging, large, and rapid. For mild cases, Yang Du Sheng Ma Tang (Yang Toxicity Corydalis Decoction) is used; for severe cases, Qing Dai Yi Wu Tang (Indigo Naturalis Single Herb Decoction) is used.

Yin toxicity (Yin Du) results from excessive Yin qi and sudden exhaustion of Yang qi. It may arise from a deficiency syndrome mistakenly treated with cold-cool herbs, or transforming into Yin toxicity after emesis or diarrhea. The patient experiences twisting pain in the umbilical and abdominal region, body pain as if beaten with a stick, cold limbs, and six pulses that are deep, minute, and weak. Yin Du Gan Cao Tang (Yin Toxicity Licorice Decoction) is the primary treatment.

"Fa Re" (Fever/Heat emission) refers to heat accumulating between the skin and becoming hot through intense dispersion. This differs from tidal fever (Chao Re), alternating chills and fever (Han Re), and heat from restlessness (Fan Zao). Heat from restlessness originates internally. Tidal fever occurs at specific times without fail. Alternating chills and fever involves chills followed sequentially by fever. In contrast, "Fa Re" can occur at any time. However, there are distinctions between "Xi Xi Fa Re" (gentle fever like feathers covering) and "Zheng Zheng Fa Re" (steaming fever). "Xi Xi Fa Re" resembles warmth under feathers, indicating heat is external; thus, Gui Zhi Tang (Cinnamon Twig Decoction) is used to disperse it via sweating. "Zheng Zheng Fa Re" resembles steaming, indicating heat is internal; thus, Tiao Wei Cheng Qi Tang (Regulate the Stomach and Order the Qi Decoction) is used to remove it via purging. Exterior-type "Fa Re" is caused by wind-cold lodging in the skin and stagnating Yang qi. Interior-type "Fa Re" is caused by Yang qi sinking into the Yin. Observing the origin of the heat clarifies whether sweating or purging is indicated. If heat starts from the skin, the pathogen is external. If heat arises internally and reaches the surface, the pathogen is internal. These two examples clarify the location of the pathogen. Since both exterior and interior present with fever, the half-exterior half-interior stage also presents with fever. Why? Because the exterior syndrome has not resolved, the pathogen has transmitted inward, but the interior has not yet formed an excess condition; this is the half-exterior half-interior state. Although the symptom is fever, some begin at the skin and gradually transform into interior heat, while others start as internal heat reaching the muscle layer. Thus, when the pathogen is in the exterior causing fever, it is exterior heat without interior heat. When the pathogen is in the interior causing fever, it is intense interior heat reaching the exterior. When in the half-exterior half-interior, both exterior and interior are heated. The *Nan Jing* states: Fever with aversion to cold indicates onset in Yang channels; absence of fever with aversion to cold indicates onset in Yin channels. However, Taiyin and Jueyin do not typically feature fever. Only in early Shaoyin disease, although fever may occur, it is distinguished by a deep pulse, diarrhea, and cold limbs; therefore, Ma Huang Fu Zi Xi Xin Tang (Ephedra, Aconite, and Asarum Decoction) is used to disperse via sweating. Furthermore, fever after sweating may indicate residual exterior pathogen, established interior syndrome, or pathogen in the half-exterior half-interior. Fever after purging may indicate incomplete removal of accumulated feces or effects from pill-based purgatives. Fever due to relapse from overexertion (Lao Fu) generally occurs because fluids are not yet restored and Qi/Blood are weak after recovery; excessive speech damages Qi, excessive thinking damages Shen (Spirit), allowing residual heat to return and the original illness to recur. Harmonizing Ying and Wei (Nutritive and Defensive systems) and clearing residual heat is appropriate. Fever due to dietary relapse (Shi Fu) requires eliminating pathogens, supporting healthy Qi, and resolving food stagnation. Although fever is common in cold injuries, in cases of simultaneous deficiency of Yin and Yang, or following diarrhea and recent sweating, fever is often dreaded. How can one apply a uniform principle?