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Zhang Fengkui stated: The manifestations of summerheat diseases are unpredictable and their onset is difficult to anticipate; they cannot be deduced by ordinary reasoning. In cases of sudden severe attack, even a physician as skilled as Bian Que would not have time to palpate the pulse or prepare decoctions. In cases of deep-seated toxicity with long-term latent infection, even Chang Sang Jun, who was said to see internal organs through the skin, could not discern the visceral pathology. Such conditions are the most difficult to detect and treat. Even in common cases of summerheat invasion, early detection is difficult, unlike Cold damage (Shanghan), which has fixed stages and syndromes that allow for diagnosis and treatment based on established patterns.

Regardless of whether the pathogen is in the exterior or interior, without following sequential transmission, and regardless of the affected Zang-Fu organs, if one is exposed to steaming toxic summerheat entering via the mouth and nose, it directly invades the Pericardium (Xinbao) and meridians. Initially, there is mental restlessness and oppression, followed by fever. Walking or sitting feels like the heat is scorching the skin and limbs, leading to tidal fever, mental irritability, and thirst. If summerheat enters the Liver, dizziness and numbness/stiffness occur; if it enters the Spleen, lethargy and unconscious sleep occur; if it enters the Lung, panting, coughing, and flaccidity occur; if it enters the Kidney, Xiao Ke (Wasting-Thirst) occurs. This does not mean only the Heart is affected while other organs are spared; rather, summerheat归于 the Heart, causing spirit confusion and sudden collapse. Summerheat damaging the muscle layer causes general restlessness, prickling pain, or redness and swelling. This is because atmospheric heat floats on the earth's surface, so human Qi also floats at the muscular exterior.

If summerheat enters the Intestines and Stomach, abdominal pain, nausea, vomiting, and diarrhea occur. Latent summerheat (Fu Shu) is similar to acute summerheat but remains hidden in the San Jiao and intestines/stomach for a long time. Heat damages Qi without damaging Form; for ten days or more, there may be no obvious symptoms, after which irregular chills and fever, cholera-like vomiting and diarrhea, abdominal distension and fullness, malaria and dysentery with irritability and thirst, abdominal pain with bloody stool, etc., emerge. Referring to *Medical Introduction* and its treatments, the primary method is clearing internal fire, supplemented by releasing the exterior.

Cold pathogens invade when the body is deficient; summerheat affects both deficiency and excess patterns, often being more severe in excess cases. Individuals with strong Qi and Blood often have latent internal fire; combined with intense external solar heat, the scorching effect is extreme. Cold resides where meridians are deficient; summerheat resides where meridians are full/excess. Cold bullies the weak, while summerheat attracts the similar (excess heat). Furthermore, those eating coarse food are often injured by cold, while those eating rich food resist cold; however, summerheat spares neither the rich nor the poor. Even living in grand mansions with piled ice, having refined constitutions and diets, wearing light silk, or dwelling in cool jade-like environments, one cannot withstand the fierce flames of summerheat.

Therefore, it is known that the toxicity of summerheat is greater than that of cold. Why did ancients specifically regard cold as a "killing/lethal" pathogen but not summerheat? Observe that Cold damage becomes critical after seven or eight days, whereas summerheat can become critical within two or three days, or even morning onset leads to evening danger, and evening onset leads to morning danger. Some strike suddenly within moments, leaving no time for rescue, such as summerwind or dry cholera. Thus, the lethal nature of summerheat is clearly more severe than that of cold. Moreover, summerheat syndromes are diverse: involving heat, fainting, internal, or external aspects. Severe cases lead to Jue (collapse), wind, epilepsy, immediate diarrhea, cholera, dry cholera, or later malaria, dysentery, and sores. There are about ten categories of diseases caused by summerheat; thus, its lethal nature is certainly not less than cold.

Except for sudden attacks and late-onset latent cases which are hard to measure, other infections have gradual progress. In midsummer, heat toxins stagnate and steam. Whether from movement or stillness, upon initial entry, people are unaware. External flowing fire and internal Yang Qi clash suddenly. If internal Yang resists, one first experiences drowsiness, fatigue, and exhaustion. When fire and Qi combine, if Qi cannot overcome Fire, the fire power strengthens, dispersing as external heat, burning continuously. Qi is consumed and Blood dries up, leading to dryness, thirst, obstruction, abdominal pain, and various malignant symptoms. These changes vary in duration; people do not realize it, and doctors may fail to distinguish it. Treatment applied only after the disease is deep makes rapid recovery difficult. Early differentiation and early treatment yield quick results and easier recovery.