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All those who are injured by overexertion, epidemic diseases, or hunger belong to the constitution of internal deficiency. This is consistent with the principle that "pathogenic factors only injure the deficient and do not injure the solid."

As observed in *Weisheng Fang* (Health Preservation Formulas): Northerners residing in Lingnan or traveling through its routes all suffer from the conflict between Yin and Yang Qi. However, among residents, only two or three out of ten fall ill, whereas among travelers, eight or nine out of ten fall ill. This is precisely because residents live quietly, while travelers suffer from exertion and injury.

*Liren Sanmi* (The Three Mysteries of Life), in its discussion on malarial fevers due to miasma, states: "If diet is regulated and daily routine is regular, pathogenic factors cannot cause harm." In areas where roads are rugged, population is sparse, water and food are unclean, and alcoholic beverages or roasted foods are often fishy, dietary habits and daily routines inevitably deviate from moderation. Moreover, there is the influence of conflicting Yin and Yang climatic Qi. Thus it is said: Miasma Qi infects only those who are exhausted by labor and injured by hunger for this reason. Furthermore, anyone residing in Lingnan must be cautious about their daily routine, moderate their diet, reduce desires, and keep the mind pure. Even if wind-pathogens invade, they cannot cause harm. When the internal environment remains stable, external pathogens cannot enter; this is a natural law. Those who become ill due to infection simply failed to exercise caution.

Lingnan is known as a hot region and borders the sea; the terrain is low, and the soil is shallow. In hot regions with shallow soil, Yang-warm Qi often dissipates outward; bordering the sea with low terrain, Yin-damp Qi often prevails. The conflict between these two Qi gives rise to cold and heat. Due to the dissipation of Yang Qi, there is no frost or snow in winter, and flowers bloom throughout the four seasons. People living there tend to have Qi stagnating upward and excessive sweating, leading to loose interstices (pores). This occurs because Yang Qi fails to return to its root. Due to the prevalence of Yin Qi, mornings and evenings are foggy, spring and summer bring incessant rain, and humid conditions persist for more than half the year. Even during the Sanfu days (the hottest period), it may not feel very hot; continuous rain in midsummer can make it chillingly cold. Food, clothing, and medicines often develop mold. People living in such environments mostly contract dampness, suffering from heavy limbs and fatigue, and frequently suffer from Hong Kong foot (Jiaoxi disease/foot rot). This results from the predominance of Yin Qi. Since Yin and Yang Qi are imbalanced and conflicting, people consequently contract diseases characterized by uneven cold and heat.

Furthermore, since Yang-warm Qi dissipates, the body's original Qi becomes insecure. Yang Qi fails to descend and floats upward, causing patients to experience oppression in the upper epigastrium and empty irritability in the chest. Since Yin-damp Qi prevails, the lower body becomes cold. Yin Qi fails to ascend and sinks downward, causing patients to experience heaviness and pain in the waist and knees, and coldness and cold extremities in the legs and feet. Observing miasma diseases in Lingnan, although symptoms may vary, the vast majority present as Shang Re Xia Han (Upper Heat and Lower Cold) due to the failure of Yin and Yang Qi to ascend and descend properly. Given that the human body naturally has Yang above and Yin below, contracting this discordant Qi of Yang-warmth and Yin-dampness naturally leads to Upper Heat and Lower Cold syndromes.

The cause of illness lies in the insecurity of Yang Qi. Each episode of chills and fever is accompanied by profuse sweating. If one then administers diaphoretic formulas such as Mahuang Tang (Ephedra Decoction), Jinfei Cao San (Boilweed Powder), or Qinglong Tang (Azure Dragon Decoction) to further release the exterior, the patient will die shortly thereafter. Worse still, if one uses purgatives for chest fullness and stuffiness in a patient whose lower body is already cold, death is inevitable. Such cases represent medical malpractice.

At that time, I contracted miasma disease, and my entire family suffered severely. I treated them all with herbs that warm the middle, secure the lower, and regulate the ascending and descending of correct Qi. All ten recovered. My two servants also fell ill, suffering from chest fullness, irritability, and unconsciousness. One requested cool herbs to clear the diaphragm. Upon careful examination, I determined the syndrome was Upper Heat and Lower Cold. I administered Shengjiang Fuzi Tang (Ginger and Aconite Decoction) cooled to lukewarm temperature. Both woke up the same day, reporting a cooling sensation in the chest and diaphragm, believing it was due to cool medicine, unaware that it contained Fuzi (Aconite Lateral Root). The next day, each took one pill of Danzhu Wan (Cinnabar Pill) on an empty stomach, enabling them to consume congee. Subsequently, using Zhengqi San (Upright Qi Powder) and Pingwei San (Peace the Stomach Powder), they recovered fully. I cured over ten others similarly. Fuzi decocted with ginger can disperse, treat heat with heat, guide deficient heat downward to eliminate old cold, and consolidate Yuan Qi. For those with irritability, it should be taken cool. For patients with irritability who dislike drinking water or fear cold liquids, this indicates deficient heat rather than true heat; Jiang Fu Tang (Ginger and Aconite Decoction) is appropriate. Shen Cunzhong (Shen Kuo) used Qizao Tang (Seven Jujube Decoction) to treat miasma, which is similar and effective after one dose. Some used Shufu Tang (Atractylodes and Aconite Decoction) but the condition worsened, likely because Atractylodes and Aconite together consolidate heat rather than disperse it. Only single-herb Fuzi is most excellent. However, if the pulse and syndrome clearly indicate non-Upper Heat/Lower-Cold patterns, such as yellow/red eyes, Fuzi should not be used. If the pulse is floating, surging, and rapid, with alternating chills and fever and no sweat, this is a Xiao Chaihu Tang (Minor Bupleurum Decoction) syndrome. If the syndrome is doubtful and chills/fever are indistinguishable, Jiahe San (Harmonizing Grain Powder) is suitable. If heat predominates, take it cool. Jiahe San regulates middle Qi, ascends and descends Yin and Yang, treats lower deficiency with middle fullness, and prevents changes in seasonal epidemics and cold damage. It resolves even heatstroke and Yang-type cold damage. It is especially suitable if cold predominates. After two to three doses, the nature of chills and fever becomes clear, allowing for symptomatic treatment without fail. Generally, Lingnan is low and damp, and people consume betel nut, which makes Qi diffuse but not solid. With sweating throughout the year, diaphoretics should not be used further; this logic is clear. Although some cases require diaphoresis or purgation, they are rare. Wise practitioners should observe this carefully.