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Wherever pathogenic factors reside, there are "traveling pathogens" (Xing Xie) and "latent pathogens" (Fu Xie); thus, treatment methods vary in difficulty, and therapeutic effects vary in speed. If it is a traveling pathogen, such as typical Shanghan (Cold Damage) starting from the Taiyang channel (Bladder meridian, governing the exterior), or transmitting to the Yangming channel (Stomach meridian), or to the Shaoyang channel (Gallbladder meridian), or entering the Stomach fu-organ from the three Yang channels, it is like a traveler passing through a place without roots, relying on its floating nature. Although the disease presentation may be severe, if the pathogen is truly in the channels, sweating therapy can resolve it; if it has truly entered the Stomach, purgation can cure it, and efficacy is achieved immediately upon administration of the medicine.

For those that are latent first and then travel, this refers to the pathogenic factors of Wenyi (Epidemic Febrile Disease), which lie latent in the Membrane Source (Moyuan, a location between the interior and exterior). Like birds nesting or beasts hiding in caves, they are beyond the reach of Ying and Wei Qi (Nutritive and Defensive Qi), and inaccessible to drugs. When they erupt, the pathogenic toxins gradually expand, invading the fu-organs internally and flooding the channels externally, damaging Ying and Wei Qi, causing various symptoms to manifest gradually, only then can treatment proceed. During the period of gradual infiltration, while the pathogenic toxin remains in the Membrane Source, one can only employ draining and promoting methods to facilitate the exit of the latent pathogen. Once the pathogenic toxin leaves the Membrane Source, observe its changes: whether it emerges to the exterior or enters the interior, and only then can one guide the pathogen out. The disease is cured only when all pathogens are eliminated. At the initial onset, when the toxic force is intense, success relies entirely on medicinal intervention. Hence, the proverb says: "Do not treat only the head symptoms in Shanghan, nor focus solely on the end stage in consumptive diseases." If it were merely Shanghan initially affecting the muscle-surface, involving only superficial pathogens in the channels, a single sweating treatment would resolve it; what difficulty would there be? This statement actually refers to Wenyi. Therefore, when epidemic pathogens are just rising, their momentum is unstoppable; the key is to make the pathogenic toxins leave the Membrane Source quickly. The crux of treatment lies in subsequent efforts: accurately identifying exterior/interior, deficiency/excess, and detailing severity and urgency. If prescriptions are administered without error, one can achieve complete success in every case. Even for the most severe infections, following these methods ensures survival. However, for patients with chronic debility reaching exhaustion, depleted by alcohol and sexual indulgence, or elderly individuals whose vitality is like a candle in the wind, their innate true Qi is nearly extinguished. Adding Wenyi makes them difficult to sustain; such cases cannot be compared to ordinary ones.

**Tongue coating changing from white to yellow:** When the pathogen is in the Membrane Source, the tongue shows a white coating; when the pathogen is in the Stomach (Wei Jia), the tongue shows a yellow coating. An "old" coating turns dark brown (like agarwood color). A white coating indicates contraindication for purgation, whereas a yellow coating indicates suitability for purgation.

**Black tongue coating:** Pathogenic toxins in the Stomach steaming upward produce a black coating. Some yellow coatings become old and turn burnt-colored; some have moist fluids appearing as soft black coatings; some are dry and hard appearing as black coatings. Two to three days after purgation, the black coating skin naturally sheds. Another type presents as a completely black tongue without any coating; this relates to channel Qi and is not an indication for purgation. It is common in pregnancy and Yin-patterns, neither being indications for purgation. After purgation, if interior signs are gone but the tongue remains black, the coating skin has not yet shed; do not purge again. Only purge when clear indications exist. If the tongue lacks coating and there are no purgative indications, mistaken purgation may cause the tongue to crack; if blackness appears, the condition is critical, requiring urgent tonification.

**Thorn-like projections on the tongue:** Heat damaging body fluids indicates the most severe form of epidemic toxin, requiring urgent purgation. In elderly patients with mild epidemic patterns lacking purgative indications, dryness on the tongue easily generates thorn-like projections. Use Shengmai San (Pulse-Generating Powder) to generate fluids and moisten dryness, causing the thorns to disappear spontaneously.

**Cracked tongue:** Due to prolonged failure to purge, blood becomes extremely depleted, often resulting in this sign. Also seen in "heat binding with side-flow" (constipation with liquid diarrhea) where untreated over time leads to fluid depletion below and rampant toxic fire above. Urgent purgation is required, and cracks will heal.

**Short, stiff, or curled tongue:** All indicate pathogen excess and true Qi deficiency. Urgent purgation is needed; once pathogens are removed and true Qi returns, the tongue relaxes naturally.

**White sandy coating:** White coating on the tongue, dry and hard like sandpaper, also called "crystal coating." It arises when fluids dry up during the white-coating stage. Even if the pathogen enters the Stomach, it fails to turn yellow. Urgent purgation is indicated.

**Moist white coating:** Indicates pathogen in the Membrane Source. Mild pathogen yields mild coating; strong pathogen yields a coating like accumulated powder covering the whole tongue. Purgation is contraindicated here. If the coating color does not change over time and other purgative indications appear, administer Sanxiao Yin (Three-Clearing Decoction); by the next morning, the tongue typically turns yellow.

**Dry/cracked lips, scorched lip color, peeling lip skin, foul breath, nostrils resembling soot:** These result from heat in the Stomach and generally require purgation. For peeling lip skin, cross-reference with other symptoms. Blackened nostrils indicate gastric epidemic toxin; purgation is unequivocally indicated. If additional purgative signs exist, purge. Post-purgation, as pathogens leave and stomach harmony returns, thirst subsides. Using herbs like Tian Hua Fen, Mai Dong, or Zhi Mu hoping to generate fluids and stop thirst is fundamentally erroneous. If profuse sweating occurs with a long, surging pulse and thirst, purgation is contraindicated; use Baihu Tang (White Tiger Decoction). As sweating continues, body temperature cools, and thirst stops.

**Red eyes, dry throat, breath hot as fire, dark red/black urine painful in drops, extremely foul stool, restlessness with flailing limbs, deep and rapid pulse:** All indicate extreme internal heat; purgation is unequivocally indicated.

**Tidal fever:** Occurs when pathogen is in the Stomach; purgation is suitable. However, some cases are unsuitable for purgation; see detailed discussions under "Interior-like but not Interior," "Heat entering the Blood Chamber," and "Mental confusion with delirium due to Qi deficiency."

**Stomach Excess (Wei Jia Shi):** Difficulty breathing, chest/diaphragm fullness/stuffiness, desire to direct Qi downward.

**Fullness beneath the heart, lump-like elevation beneath the heart, pain beneath the heart, abdominal distension/pain worsening on pressure, distending pain beneath the heart:** All indicate pathogenic excess in the Stomach with internal Qi blockage. Purge to restore Qi flow.

**Headache and distending pain:** Caused by Stomach Excess preventing Qi descent. Purging stops headache immediately. If headache occurs at onset without other purgative signs, do not purge.

**Urinary retention:** Constipation causes Qi stagnation. Once bowels move, urination resolves immediately. Mistaken use of Qi-moving or diuretic herbs is ineffective.

**Constipation with extremely foul flatus:** Strong purgative indications exist; purgation is unequivocally indicated. In cases of blood depletion without exterior/interior signs, indicating deficiency-dryness, use Honey Suppositories or Gallbladder Suppositories (external evacuation methods).

**Large Intestine Gel-like Obstruction:** Patients usually have loose stools. If epidemic pathogens transmit inward, feces become extremely foul, sticky like glue, never solidifying, becoming increasingly sticky and obstructed. This prevents Stomach Qi from descending and traps epidemic toxin. Without purgation, death ensues. Removing the gel-like obstruction eliminates purgative indications and cures the disease.

**Diarrhea with協热 (Collateral Heat) and Heat Binding with Side-Flow:** Both require purgation. See details under the "Stool" section.

**Si Ni (Four Extremities Cold), Mai Jue (Pulse Absence), Ti Jue (Body Cold):** All belong to Qi closure, with Yang Qi constrained internally unable to circulate outward, caused by Stomach Excess. Purge. If these signs appear *after* purgation, it indicates collapse/deficiency; use tonification.

**Manic behavior:** Caused by Stomach Excess and abundant Yang Qi; purgation is indicated. Differentiate from false mania due to deficiency-heat or mania arising from attempted sweating. See specific sections; purgation is contraindicated in those cases.