张凤逵曰。暑证变幻无常。入发难测。不可寻思。彼暴中之激烈。扁鹊不及 指而
投咀。久伏之深毒。长桑不能隔肤而见脏。最为难察而难救已。即寻常之感。亦难
于知觉。非若伤寒之有定期定证。可据可疗者。不拘表里。不以渐次。不论脏腑。冒
暑蒸毒。从口鼻入者。直中心胞经络。先烦闷。后身热。行坐近日。熏烁皮肤肢体者。
实时潮热烦渴。入肝则眩晕顽麻。入脾则昏睡不觉。入肺则喘咳痿 。入肾则消
渴。非专心主而别脏无传入也。中暑归心。神昏卒倒。暑伤肉分。周身烦躁。或如针
刺。或有赤肿。盖天气浮于地表。故人气亦浮于肌表也。冒暑入肠胃。腹痛恶心呕
泻。伏暑即冒暑。久而藏伏三焦肠胃之间。热伤气而不伤形。旬日莫觉。变出寒热
不定。霍乱吐泻。膨胀中满。疟痢烦渴。腹痛下血等。自入肝至此。采医学入门并主
治法。皆以清内火为主。而解表兼之。寒之中人。乘其虚。暑则虚实并中。而实更剧。
盖气血强盛之人。内已有伏火。加之外火炎炎相合。故焦灼为甚。经虚处寒栖之。
经实处暑栖之。寒凌其弱。而暑亲其类也。又藜藿常被寒。惟膏粱独能御。若暑则
不问膏粱藜藿。而咸能胜之侮之。虽广厦累冰。蕙质生粟。轻罗纨绮。泠泠玉树。一
犯其烈焰。讵能却之乎。是以知暑气之毒盛于寒。乃古人专以寒为杀厉之气。而
不及暑何也。试观伤寒至七八日方危。暑病则有危在二三日间。甚至朝发暮殆。
暮发朝殆。尤有顷刻忽作。拯救不及者。如暑风干霍乱之类。然则暑之杀厉之气。
视寒尤甚。彰明较着矣。且暑证多歧。中热中 中内中外。甚者为厥为风。为颠痫。
即发则泄泻霍乱干霍乱。积久后发则疟痢疮疡。种种病名。约有十余科。皆暑为
厉。则杀厉之气。视寒岂少哉。除暴中暴发。久伏后发。不可度量。其余受发。亦有渐
次焉。盖盛夏之时。热毒郁蒸。无论动得静得。其初入人也。不识不知。外之流火与
内之阳气。骤遇而争。阳气不服。先昏 倦疲。及火与气合。气不能胜。火力渐强。散
为外热。烧灼不已。气耗而血枯。故燥渴痞塞腹痛。诸恶证作焉。此其变化。或乍或
久。人莫自觉。医家亦不能辨。至病深而后施治。故难速愈。宜早辨而早治之。则取效速而易愈。
张凤逵说：暑病的证候变化无常，发病难以预测，不可用常理去揣度。那些突然中署的剧烈情况，即使是扁鹊那样的神医也来不及切脉诊断和抓药煎服；那些潜伏日久、毒性深重的情况，即使是长桑君那样能隔肤见脏的神人也无法看清内脏病变。这类病症最为难以察觉且难以救治。即便是普通的感受暑邪，也难以被及时察觉，不像伤寒那样有固定的病程规律和典型证候，可以依据进行诊疗。
无论病位在表在里，也不按循序渐进的传变规律，更不论涉及哪个脏腑，只要冒犯暑热毒气，从口鼻吸入，就会直接侵犯心包和经络。起初表现为心烦闷乱，随后出现身体发热。行走坐卧时，感觉热气熏烤皮肤肢体，随即出现定时潮热、心烦口渴。如果暑邪侵入肝脏，则出现眩晕和肢体麻木僵硬；侵入脾脏，则昏睡不醒；侵入肺脏，则气喘咳嗽、肢体痿软；侵入肾脏，则出现消渴症。这并不是说只有心脏受病而其他脏腑不受传变，而是因为中暑归于心，导致神志昏迷、突然跌倒；暑邪损伤肌肉分肉，导致周身烦躁，或如针刺般疼痛，或有红肿。这是因为天气浮于地表，所以人体之气也浮于肌表。
如果暑邪侵入肠胃，会出现腹痛、恶心、呕吐、腹泻。伏暑即是冒暑，但潜伏时间较长，藏伏在三焦和肠胃之间。暑热伤气而不伤形体，经过十天半个月可能没有明显感觉，之后变出寒热不定、霍乱吐泻、腹胀中满、疟疾痢疾伴心烦口渴、腹痛便血等症状。以上从肝脏到肾脏的症状，参考《医学入门》及其主治方法，治疗都以清泄内火为主，兼以解表。
寒邪伤人，是乘人体正气虚弱之时入侵；而暑邪则是虚实之体都会受到侵害，且实证患者病情往往更重。因为气血强盛的人，体内原本就有伏火，再加上外界炎炎烈日之火相合，所以焦灼程度非常严重。经气虚弱之处，寒邪栖息于此；经气充实之处，暑邪栖息于此。寒邪欺凌虚弱者，而暑邪亲近同类（实热者）。此外，粗茶淡饭之人常被寒邪所伤，只有富贵膏粱之人才能抵御寒邪；但对于暑邪，无论富贵还是贫贱，都能战胜并侮辱它。即使住在广厦之中堆积冰块，体质如蕙草般高洁、饮食如粟米般精细，穿着轻罗纨绮，身处如玉树般清凉的环境，一旦遭遇那烈焰般的暑气，怎么能抵挡得住呢？
因此可知，暑气的毒性比寒邪更盛。为什么古人专门将寒视为杀厉之气，却不及暑邪呢？试看伤寒病到了七八天才危险，而暑病则在两三天间就有生命危险，甚至早晨发作晚上就危殆，晚上发作早晨就危殆，更有顷刻间突然发作，抢救不及的情况，比如暑风、干霍乱等。那么暑邪的杀厉之气，比起寒邪更为严重，这是显而易见的。而且暑病的证候多种多样，有中热、中暍、中内、中外之分，严重的会导致厥证、风证、癫痫。刚发作时就出现泄泻、霍乱、干霍乱，积久后发作则出现疟疾、痢疾、疮疡等。各种病名约有十余科，都是暑邪为害，那么它的杀厉之气，难道比寒邪少吗？
除了暴中和暴发、久伏后发这两种情况无法度量外，其余的受邪和发病，也是有渐进过程的。大概在盛夏时节，热毒郁蒸，无论是动得还是静得，当暑邪初入人体时，人们不知不觉，外界的流火与体内的阳气骤然相遇而争斗，体内阳气不服，先出现昏沉、倦怠、疲乏。等到火与气结合，气不能胜，火力逐渐增强，散发为外部热象，烧灼不停。气耗损而血枯竭，所以出现燥渴、痞塞、腹痛等各种恶证。这些变化，有的短暂有的持久，人们自己不能察觉，医生也不能辨别，直到病深入后才进行治疗，所以难以迅速痊愈。应当尽早辨别并尽早治疗，这样才能见效快且容易痊愈。
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.