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Whenever fever occurs, one must examine whether there are accompanying syndromes in the exterior (Biao) or interior (Li). Only then can the root cause of the fever be thoroughly investigated. If there is merely fever without any other accompanying syndromes in the exterior or interior, this belongs to internal injury fever (Nei Shang Fa Re), which is not covered in this context. The so-called accompanying syndromes refer to external symptoms such as headache, aversion to cold (E Han), body pain, and lower back pain, indicating that the pathogen is in the exterior; or internal symptoms such as irritability, thirst, chest and abdominal pain, constipation, or diarrhea, indicating that the pathogen is in the interior. Fever is merely the initial manifestation; one must cross-reference and clarify the diagnosis to formulate an accurate treatment plan.

In discussing fever, emphasis is always placed on the Taiyang (Greater Yang) channel, because the Taiyang governs the exterior, controls Ying (Nutritive Qi) and Wei (Defensive Qi), and serves as the master qi of all Yang channels. The Classic states: "Yang protects the exterior and maintains solidity." External pathogens often attack due to the insecurity of Wei Qi. Therefore, when discussing exogenous diseases, one always begins with the Taiyang channel. However, although Taiyang governs the exterior, its root originates from Zhi Yin (referring to the Foot-Shaoyin Kidney channel). This reflects the mutual resonance between the interior and exterior, upper and lower. Thus, from Taiyang transmitting to Yangming (Stomach), Shaoyang (Gallbladder), and the Three Yin channels, fever syndromes are intermixed and detailed below.

The disease of the Taiyang channel presents with a floating pulse (Mai Fu), stiffness and pain in the head and nape, and aversion to cold. Fever does not occur immediately. The aversion to cold is the mechanism preceding fever. The "Taiyang Disease" mentioned in the Treatise refers specifically to this pulse and these symptoms. Diseases of other channels follow this principle.

Taiyang Wind Strike (Zhong Feng) presents with fever and sweating, with a floating and moderate pulse. Wind damages Wei Qi; Guizhi Tang (Cinnamon Twig Decoction) should be used to resolve the muscle layer. Taiyang Cold Damage (Shang Han) presents with fever and no sweating, with a floating and tight pulse. Cold damages Ying Blood; Mahuang Tang (Ephedra Decoction) should be used to induce sweating. However, for the Guizhi Tang syndrome, one must drink hot thin porridge to assist the medicinal power, aiming for a slight, uniform perspiration, not allowing sweat to flow profusely. In contrast, the Mahuang Tang syndrome aims to induce heavy sweating. These are two distinct methods for treating Wind and Cold respectively using Mahuang Tang and Guizhi Tang, which cannot be confused. If both Wind and Cold injure simultaneously, affecting both Ying and Wei, the Classic states: "In Taiyang Wind Strike, if the pulse is floating and tight, with fever, aversion to cold, body pain, no sweating, and irritability, Da Qing Long Tang (Major Blue Green Dragon Decoction) is the primary treatment." It also states: "In Cold Damage, if the pulse is floating and moderate, the body is not painful but heavy, occasionally feeling lighter, and there are no Shaoyin signs, Da Qing Long Tang is used to disperse." Generally, Wind Strike pulses should be floating and moderate, yet a floating and tight pulse appears; Cold Damage pulses should be floating and tight, yet a floating and moderate pulse appears. This alters the standard methods of Guizhi and Mahuang, leading to the primary use of Da Qing Long Tang to induce heavy sweating. This is Zhang Zhongjing's method for treating Wind and Cold, with established formulas available. Some claim that Zhang's methods are detailed for Wind and Cold but brief for Warmth (Wen Re); this is incorrect. Outside of Wind and Cold, Zhang explicitly identifies Taiyang disease with fever, thirst, and no aversion to cold as Warm Disease (Wen Bing). This is the most crucial key to differentiation. The difference from Wind and Cold lies entirely in "thirst" and "no aversion to cold." Wind and Cold fever must include aversion to cold; the absence of aversion to cold here requires differentiation. Wind and Cold fever involves no initial thirst; thirst only arises after transmission. Here, thirst appears at the onset, requiring further differentiation. There is also what is called Damp-Warmth (Shi Wen). The Classic says: "For patients with Dampness, the whole body aches, there is fever, and the body color resembles smoke-stained yellow." It also says: "In Taiyang disease, joint pain with irritability and a sinking, fine pulse is named Damp Bi (Dampness Obstruction)." Here, Zhang mentions Dampness but not Warmth. However, by adding the symptoms of fever, thirst, and no aversion to cold to the various Dampness syndromes listed in the Treatise, the characteristics of Damp-Warmth can be deduced by analogy. As for Heat Disease, Zhang clearly states: "Sun Heat in Taiyang is Shu (Summerheat). The person sweats, has aversion to cold, body heat, and thirst." This shares thirst with Warm Disease, but Shu includes aversion to cold while Warm Disease does not, marking a distinction. Zhang treats it with Renshen Baihu Tang (Ginseng White Tiger Decoction), providing both method and formula. This is the general method for differentiating fever at the onset of Taiyang disease, representing the practical foundation for treating Shanghan (Cold Damage).

Some describe "Xi Xi Fa Re" (fever like feathers covering, mild warmth) as purely exterior without interior involvement. Others describe "Zheng Zheng Fa Re" (fever steaming from within) as originating from the interior and reaching the exterior. Taiyang governs the exterior; its fever must accompany head/nape stiffness, body pain, etc., which are deployments of Taiyang, purely exterior. Yangming fever accompanies irritability, fullness, and thirst. Shaoyang fever accompanies chest/hypochondriac fullness/pain, bitter taste, and desire to vomit. Their pulses are all primarily floating. Taiyang pulse may be floating-moderate or floating-tight; Yangming pulse is floating-large; Shaoyang pulse is floating-wiry. If the pulse is sinking and not floating, fever syndromes also appear among the Three Yin diseases. Although the fever is the same, the causes differ; one cannot assume the pathogen is in the exterior simply because of fever and treat all according to Taiyang exterior-releasing methods.

Yangming fever differs from Taiyang. The Classic asks: "What are the external manifestations of Yangming disease?" Answer: "Body heat, spontaneous sweating, no aversion to cold, but aversion to heat." Initially, there is aversion to cold and fever; now aversion to cold ceases spontaneously. Even with sweating, if heat persists, it indicates transformation into Yangming. At this stage, regardless of whether the pathogen was Wind, Cold, Summerheat, or Dampness, it ultimately transforms into Fire. For Yangming disease, Zhang Zhongjing prohibits sweating. Treatment should distinguish between Channel (Jing) and Fu organ syndromes. If heat is severe but still in the Channel, sweet-cold herbs should directly clear the heat. As stated: "After vomiting or purging in Cold Damage, if unresolved after 7-8 days, heat binds internally, causing exterior and interior heat, occasional aversion to wind, extreme thirst, dry tongue, irritability, and desire to drink several liters of water, Baihu Jia Renshen Tang (White Tiger Plus Ginseng Decoction) is used." If the pathogen has entered the Fu organ but sweating and fever persist: "Three days after Taiyang disease, if sweating fails to resolve the condition and there is steaming fever, it belongs to the Stomach; Tiaowei Chengqi Tang (Regulate the Stomach and Order the Qi Decoction) is used." Also: "In Yangming disease, if there is fever and excessive sweating, urgent purging is required; Da Chengqi Tang (Major Order the Qi Decoction) is indicated."

Shaoyang fever also carries a prohibition against sweating. Shaoyang disease typically features alternating chills and fever, but fever can belong to Shaoyang, accompanied by hypochondriac fullness, irritability, and desire to vomit, with a wiry pulse. Thus: "Four or five days after Cold Damage, if there is body heat, aversion to wind, neck stiffness, hypochondriac fullness, warm hands/feet, and thirst, Xiao Chaihu Tang (Minor Bupleurum Decoction) is used." Also: "If Cold Damage fever persists despite sweating, with epigastric hardness, vomiting, and diarrhea, Da Chaihu Tang (Major Bupleurum Decoction) is used." Hypochondriac pain, vomiting, and thirst clearly indicate Shaoyang half-exterior/half-interior syndromes. Even if exterior fever remains, one must not use Taiyang exterior-releasing methods, but treat the half-exterior/half-interior with Xiao or Da Chaihu Tang. Furthermore: "If the pulse is wiry and fine with headache and fever, it belongs to Shaoyang. Shaoyang must not be induced to sweat; doing so leads to delirium, indicating stomach disharmony. If the stomach harmonizes, recovery occurs; if not, irritability and palpitations result." Wiry is the definitive pulse of Shaoyang; fine indicates the pathogen is not in the exterior. Zhang warns against mistaking headache and fever for Taiyang, clarifying it as Shaoyang and reiterating the prohibition against sweating. The first two points differentiate Shaoyang fever by symptoms; this point differentiates by pulse.

Taiyin fever presents with a sinking, non-floating pulse, accompanied by vomiting, diarrhea, and abdominal pain. The Classic asks: "Fever, aversion to cold, headache, body pain, vomiting, and diarrhea—what disease is this?" Answer: "This is Huoluan (Cholera-like disorder). Huoluan involves spontaneous vomiting and diarrhea; if diarrhea stops and fever returns..." Vomiting and diarrhea belong to Taiyin. Fever returning after diarrhea stops indicates Yin transforming into Yang. Having transformed to Yang, the pulse should become floating. Hence: "If Taiyin disease pulse is floating, sweating may be induced; Guizhi Tang is appropriate." After vomiting and diarrhea, fluids are damaged, often leading to thirst and fever. Hence: "For Huoluan with headache, fever, body pain, more heat than cold, and desire to drink, Wuling San (Five Ingredient Powder with Poria) is used. If more cold than heat and no desire to drink, Lizhong Tang (Regulate the Middle Decoction) is used." This differs from Yang channel fever treatments.

Shaoyin fever is easily confused with Taiyang. Thus, for Da Qing Long Tang, Zhang insists on excluding Shaoyin signs before inducing heavy sweating. He emphasizes: "Shaoyin disease with sinking, fine, rapid pulse is interior; do not sweat." Also: "Shaoyin disease with faint pulse must not be sweated, as it causes Yang collapse." Thus, Shaoyin forbids sweating. Yin diseases must transform to Yang. "Shaoyin disease with vomiting, diarrhea, non-cold limbs, and return of fever indicates survival." Taiyang Cold Water and Shaoyin Ministerial Fire complement each other. Here, Shaoyin is suppressed by Cold; receiving Yang heat allows Qi to return to Taiyang, hence fever without death. How to identify Shaoyin fever? The Classic states: "Shaoyin disease presents with sinking, fine pulse and desire to sleep." Sinking/fine is the definitive pulse; desire to sleep is the definitive symptom. Unlike Taiyin where limbs are warm, Shaoyin has cold limbs, plus aversion to cold and curling up. Severe cases have cold limbs beyond elbows/knees, no pulse, sore throat, irritability, or pus/blood in stool. Life hangs in the balance. Shaoyin Kidney stores Water and Fire, the root of congenital essence, leading to complex cold-heat patterns. Fever implies Yang heat within Yin cold. Forcing sweating in Shaoyin destroys the root, leading to lower Jue and upper exhaustion, which is difficult to treat. If Cold directly invades Shaoyin causing immediate fever: "Shaoyin disease, just acquired, with return of fever and sinking pulse, uses Mahuang Fuzi Xixin Tang (Ephedra, Aconite, and Asarum Decoction)." Also: "Shaoyin disease, 2-3 days later, uses Mahuang Fuzi Gancao Tang for mild sweating, as there are no interior signs." Fuzi stabilizes Kidney True Yang to secure the root before expelling pathogens. This mild sweating contains the meaning of not sweating.

Jueyin fever is judged by the ratio of Jue (cold extremities) to Re (fever) to determine disease progression. Jueyin means the end of two Yins. Within this end lies the generation of Yang. When Yin and Yang fail to connect smoothly, Jue occurs. Equal Jue and Re indicate recovery; more Jue than Re indicates progression; more Re than Jue indicates regression. Jue refers to cold extremities. Extreme cold beyond elbows/knees is called Jue. Here, fever serves as a thread of surviving Yang, indicating impending recovery. "Shanghan starting with Jue, followed by fever and diarrhea, will see diarrhea stop." "Shanghan with little heat and slight Jue, desiring food, indicates recovery." If fever and severe diarrhea persist with unending Jue, death ensues. "Six or seven days after Shanghan, if no diarrhea initially, but sudden fever and diarrhea with endless sweating, death ensues." After fever, the pulse must rise from sinking to floating, indicating Yin transforming to Yang. "Slightly floating pulse indicates recovery; non-floating indicates otherwise." This aligns with Shaoyin fever principles.

Shanghan pulses remain quiet/still throughout. "Two or three days after Shanghan, if the pulse is quiet, it does not transmit. If rapid and urgent, it transmits." The Classic states: "If sweating occurs but the pulse remains restless, death results." Initial fever heals if the pulse is quiet; post-transmission healing requires a quiet pulse. Diseases moving from exterior to interior start with Taiyang; those moving from interior to exterior start with Jueyin. Taiyang governs the exterior, connects to Fengfu, roots in Zhi Yin, and pairs with Shaoyin. Zhang refrains from reckless sweating due to this. Three Yin diseases forbid sweating. Even Yang channel fevers may not originate from Taiyang. Onset with thirst, sweating, and constipation suggests Yangming transmitting to Taiyang. Onset with vomiting and hypochondriac pain suggests Shaoyang transmitting to Taiyang. One cannot cite "Day one, Taiyang receives the pathogen" to justify universal exterior release.

Warm diseases mostly originate in Yangming. Damp-Warmth pathogens lodge in Mu Yuan (Omentum/Source), a half-exterior/half-interior location. Fever accompanies chest oppression, abdominal fullness, irritability, thirst, and constipation, plus bone/joint pain and a coating like accumulated powder. The disease attacks Shaoyang and Taiyin first, connecting internally to the Stomach Fu and externally overflowing to Taiyang. Damp patients sweat easily; heavy sweating must be avoided. "Yi Qi" (Pestilential Qi) is not a seventh Qi outside the Six Qi. The Six Qi are natural seasonal forces; excess leads to pestilence, causing contagious illness with similar symptoms across ages. Treatment still adheres to the Six Qi principles.

Fever recurring after recovery is called "Yi Re" (Lingering Heat), caused by improper diet or activity. Post-illness diet should be light to restore Stomach fluids and eliminate pathogens, allowing normal appetite. Eating rich/fatty foods or using tonics is inadvisable. The Classic warns: "When heat subsides slightly, eating meat causes relapse; overeating causes lingering heat." Recurrent fever can be Deficient or Excess. The Classic advises: "Assess deficiency/excess, regulate reversal/normalcy, ensuring cure." Zhang's methods for post-meal labor recurrence or post-major illness care are detailed in the Treatise, requiring no external sources.