夫水润下，火炎上，自然之理也。三焦者，水谷之道路，津液者，乃气之精化，流
通三焦，以制火者也。今口干而渴者为气虚，火盛津液枯竭也。《经》曰∶肝热则口
酸，心热则口苦，脾热则口甘，肺热则口辛，肾热则口咸，或口淡者，是胃热也。若
渴者，乃五脏之热，而火之使然，然火之为用，非虚不发，发而不解，则津液不能上行
以制火。火乃炎上，熏灼心脾，是以津液为之下陷，华池为之干涸，因而为渴。渴者，
脏腑精华燥槁也，其治为宜除热润燥，气化则津液自生，热除则烦渴自已。然痘前渴
者，宜疏解清化，痘出而内热自除，或紫苓汤加干葛、荆芥。若痘后渴者，则补养气
血，用保元汤加麦冬、五味之类。如泻者，则用参苓白术散主之。若阴虚火动而渴者，
最为难疗。夫阴虚者，血虚也。血虚不能骤补，盖血与气大有不同，气，无形之物，
血，有形之物，无形者有神，卒能旺于斯须，有形者无神，须当养于平素耳。故治宜
与地黄丸，加肉桂、五味，既补肾阴，且使釜下有火，则锅盖自然不燥，诚为虚渴之圣
药也。
凡痘疮发渴者，不可饮水，否则，津液不行，燥渴愈甚，且疮靥后，其痂迟落，或
生痈肿矣。盖脾胃属土而恶湿，喜湿而恶
寒，外主肌肉，若饮冷水，则脾胃内虚，肌肉外滞，津液冰结，荣卫不周是以疮痂迟
落，或生痈肿矣。然饮有阴阳，阳盛阴虚者，则冰雪不知寒，阴盛阳虚者，则沸汤不
知热，故发热作渴，手足逆冷，大便自利。喜饮热汤者，是阴盛阳虚也，治宜补阳。若
发热作渴，大便秘结，手足并热，喜饮冷水者，是阳盛阴虚也，治宜补阴。若烦热作
渴，面赤晴白，是肾经虚热也，治宜滋肾。若正靥之间，忽不能靥，头温足冷，腹胀泄
泻，气促烦渴者，此虚寒之甚也，急进参术桂附之类。切忌寒凉之药，及诸蜜水瓜果，
否则，津液收敛，转生焦渴，冷气内攻逼阳于上，愈加腹胀喘渴泄泻而死。
凡渴多属于热，然皆由脏腑津液燥槁，实非有余也。至若腹胀渴者，或泻渴者，或
足指冷渴者，或惊悸渴者，或身温渴者，或身热而面 白色渴者，或寒战而渴不止者，
或气急切牙渴者，或饮水而转渴不已者，以上九症，尤非实热，宜急温补救里，滋养津
液，以杜痒塌喘渴而死。若谓以为热症而治之，危亡立见矣。
痘际渴者，常有症之也。但有应不应之候，在二三朝间身热口渴者，此是毒症于
里，热邪熏灼，治宜不托。在四朝以后，身热口渴者，此是津液外泄，化为脓浆，治宜
参，此皆应候也。惟在结痂之后，则邪毒尽化，里无留邪，如反大渴者，则是真气渐
耗，火毒宜升，此不应之候也，急与解毒滋阴，生津利咽。若渐减者吉，转渴者，必变
喘胀而危矣。
凡能食而渴者，肺热也。《经》曰∶心移热于肺，传为膈消。是由心火上炎，乘于
肺金，故熏蒸焦膈，传耗津液也。其治在上焦，宜人参白虎汤加黄连主之。如不能食而
渴者，是脾虚也，盖由脾元既弱，不能为胃行其津液，其治在中焦，须防发泻，故宜参
苓白术散主之。如自利而渴者，是邪传肾也。盖自利而渴，属足少阴虚，故引水自
救。夫肾主五液，其脉络于肺，系于舌本，若邪传于肾，则开合不司，故乃自利，利则
津液下走，肾水干涸，不能上润于舌，故大渴也。其治在下焦，宜异功散以温之。
水具有向下流动的特性，火具有向上燃烧的特性，这是自然的规律。三焦是水谷运行的通道，津液是气的精微转化物，它流通于三焦之中，用以制约火势。如今出现口干且口渴的症状，是因为气虚，导致火势旺盛、津液枯竭。《黄帝内经》说：肝热则口酸，心热则口苦，脾热则口甘，肺热则口辛，肾热则口咸，若口淡无味，则是胃热所致。如果感到口渴，是由于五脏有热，由火邪引起。然而火邪的作用，非因虚证不会发作；一旦发作而不能解除，就会导致津液不能上行以制约火势。火性炎上，熏灼心脾，致使津液下陷，口腔（华池）干涸，从而产生口渴。口渴，意味着脏腑的精华物质干燥枯槁，治疗应当适宜地清除热邪、滋润燥热，气化功能恢复则津液自然生成，热邪去除则烦渴自然停止。
如果在痘疹发出之前出现口渴，适宜采用疏风解表、清热化湿的方法，待痘疹发出后内热自除，或者使用紫苓汤加葛根、荆芥。若在痘疹发出之后出现口渴，则应补养气血，使用保元汤加麦冬、五味子之类。如果出现腹泻，则使用参苓白术散为主方治疗。若是阴虚火动导致的口渴，最为难治。所谓阴虚，即是血虚。血虚不能迅速进补，因为血与气大不相同：气是无形的物质，血是有形的物质；无形之物具有神机，能短时间内旺盛；有形之物缺乏神机，必须在平时进行调养。因此治疗适宜用地黄丸，加入肉桂、五味子，既补肾阴，又使锅下有火（命门之火），则锅盖自然不干，这确实是治疗虚性口渴的圣药。
凡是痘疮引发口渴的，不可饮水，否则津液运行不畅，燥渴会更加严重，且在痘疮结痂后，痂皮脱落延迟，或者发生痈肿。这是因为脾胃属土，厌恶湿气，喜欢湿润而厌恶寒冷，外主肌肉。如果饮用冷水，会导致脾胃内部虚弱，肌肉外部阻滞，津液如冰般凝结，营卫之气不能周流全身，因此痂皮脱落延迟，或者发生痈肿。
饮水也有阴阳之分：阳盛阴虚的人，即使喝冰雪水也不知道冷；阴盛阳虚的人，即使喝沸水也不知道热。所以发热作渴、手足逆冷、大便自利、喜欢喝热汤的，属于阴盛阳虚，治疗适宜温补阳气。若发热作渴、大便秘结、手足俱热、喜欢喝冷水的，属于阳盛阴虚，治疗适宜滋补阴液。若烦热作渴、面赤目白，是肾经虚热，治疗适宜滋补肾阴。若在痘疮正行期间，突然不能结痂，头温足冷，腹胀泄泻，气促烦渴的，这是极度虚寒的表现，急需服用人参、白术、肉桂、附子之类的药物。切忌使用寒凉药物以及蜂蜜水、瓜果等，否则津液收敛，反而产生焦渴，冷气内攻逼迫阳气上浮，更加重腹胀、喘促、口渴、泄泻，最终导致死亡。
凡是口渴大多属于热证，但都是由脏腑津液干燥枯槁引起的，实际上并非有余之实证。至于出现腹胀而渴、腹泻而渴、脚趾冷而渴、惊悸而渴、身体温和而渴、身体热而面色苍白而渴、寒战而渴不止、气急咬牙而渴、饮水后反而渴得更厉害的以上九种症状，尤其不是实热，应当紧急温里救逆，滋养津液，以防止因瘙痒塌陷、喘促口渴而死。如果误认为是热证而治疗，危亡立刻就会到来。
痘疹初期出现的口渴，常有相应的病机。但有应候（正常反应）和不应候（异常反应）的区别。在发病二三日之间，身热口渴的，这是毒邪在里，热邪熏灼，治疗适宜不急于托毒外出。在四日以后，身热口渴的，这是津液外泄，化为脓浆，治疗适宜用人参补气，这些都属于应候。只有在结痂之后，邪毒已经全部化解，里没有残留邪气，如果反而出现大渴，则是真气逐渐消耗，火毒宜升发，这是不应候，急需给予解毒滋阴、生津利咽的药物。如果口渴逐渐减轻为吉兆，如果转为更渴，必然变为喘胀而危及生命。
凡是能吃食物却口渴的，是肺热。《黄帝内经》说：心将热邪移于肺，传变为膈消。这是由于心火上炎，乘袭肺金，熏蒸焦膈，耗伤津液所致。其病位在上焦，适宜用人参白虎汤加黄连为主方治疗。如果是不能吃食物却口渴的，是脾虚。这是因为脾元虚弱，不能为胃传输津液，其病位在中焦，须防止发生腹泻，故适宜用参苓白术散为主方治疗。如果是自发性腹泻且口渴的，是邪气传入肾脏。因为自利而渴，属于足少阴肾经虚证，所以引水自救。肾主管五液，其脉络连接肺部，系于舌根。如果邪气传入肾脏，则开合功能失调，所以发生自利；腹泻导致津液下走，肾水干涸，不能上润于舌，所以出现大渴。其病位在下焦，适宜用异功散来温补。
Water has the nature of flowing downward, and fire has the nature of flaming upward; this is the natural principle. The Sanjiao (Triple Burner) is the pathway for water and grain, while Jinye (Body Fluids) are the refined transformation of Qi. They circulate through the Sanjiao to restrain Fire. Currently, dry mouth with thirst indicates Qi deficiency, where excessive Fire leads to the depletion of Body Fluids. The *Classic* states: Liver Heat causes sour taste in the mouth; Heart Heat causes bitter taste; Spleen Heat causes sweet taste; Lung Heat causes pungent taste; Kidney Heat causes salty taste; or a bland taste indicates Stomach Heat. If there is thirst, it results from Heat in the Five Zang Organs, driven by Fire. However, Fire does not flare up unless there is Deficiency; if it flares up and is not resolved, Body Fluids cannot ascend to restrain Fire. Since Fire flames upward, it scorches the Heart and Spleen, causing Body Fluids to sink and the "Huachi" (oral cavity/saliva pool) to dry up, thereby causing thirst. Thirst signifies the dryness and withering of the essence of the Zang-Fu organs. Treatment should appropriately clear Heat and moisten Dryness; once Qi transformation is restored, Body Fluids will generate naturally, and once Heat is removed, irritability and thirst will cease.
If thirst occurs before the eruption of smallpox (or measles/rash), it is appropriate to use methods that disperse and resolve, clearing heat and transforming dampness; after the rash erupts, internal Heat will naturally subside, or one may use Ziling Tang (Purple Talc Decoction) added with Gegen (Kudzu Root) and Jingjie (Schizonepeta). If thirst occurs after the eruption, one should tonify Qi and Blood, using Baoyuan Tang (Preserve the Origin Decoction) added with Maidong (Ophiopogon) and Wuweizi (Schisandra). If diarrhea is present, Shenyin Baizhu San (Ginseng and Atractylodes Macrocephala Powder) should be used as the main formula. If thirst is caused by Yin Deficiency with Hyperactive Fire, it is most difficult to treat.所谓 Yin Deficiency refers to Blood Deficiency. Blood Deficiency cannot be rapidly supplemented because Blood and Qi are vastly different: Qi is formless, whereas Blood is formed. Formless things possess Shen (Spirit/Divine function) and can flourish momentarily; formed things lack Shen and require nourishment over time. Therefore, treatment should employ Dihuang Wan (Rehmannia Pill), added with Rougui (Cinnamon Bark) and Wuweizi, which both tonifies Kidney Yin and ensures there is fire under the pot (Mingmen Fire), so that the lid naturally does not dry out; this is indeed the sacred medicine for treating Deficiency-type thirst.
In cases where smallpox induces thirst, drinking water is contraindicated; otherwise, the movement of Body Fluids will be obstructed, making dryness and thirst worse. Furthermore, after the scabs form, they may fall off late or abscesses may develop. This is because the Spleen and Stomach belong to Earth, dislike Dampness, prefer Moisture but hate Cold, and govern the muscles externally. Drinking cold water causes internal weakness of the Spleen and Stomach and external stagnation of the muscles, freezing the Body Fluids and preventing Ying and Wei Qi from circulating fully, thus leading to delayed scab shedding or abscess formation.
Drinking water also has Yin and Yang distinctions: Those with Excess Yang and Deficient Yin do not feel cold even when consuming ice snow; those with Excess Yin and Deficient Yang do not feel hot even when consuming boiling water. Thus, fever with thirst, cold limbs, spontaneous diarrhea, and preference for hot drinks indicate Excess Yin and Deficient Yang, treated by tonifying Yang. Fever with thirst, constipation, hot hands and feet, and preference for cold drinks indicate Excess Yang and Deficient Yin, treated by nourishing Yin. Irritability with heat, red face, and white eyes indicate Empty Heat in the Kidney Meridian, treated by nourishing the Kidney. During the peak of the rash, if sudden failure to crust occurs, with warmth in the head and cold in the feet, abdominal distension, diarrhea, shortness of breath, and irritability/thirst, this indicates severe Deficiency-Cold; urgently administer Ren Shen (Ginseng), Bai Zhu (Atractylodes), Rou Gui (Cinnamon Bark), and Fu Zi (Aconite). Strictly avoid cold-natured medicines, honey water, and fruits; otherwise, Body Fluids will constrict, turning into intense thirst, with Cold Qi attacking internally forcing Yang upward, worsening abdominal distension, wheezing, thirst, and diarrhea, leading to death.
Generally, thirst belongs to Heat, but it always stems from the dryness and withering of Zang-Fu Body Fluids, and is truly not an Excess condition. Regarding abdominal distension with thirst, diarrhea with thirst, cold toes with thirst, palpitations with fright with thirst, warm body with thirst, hot body with pale face and thirst, chills with unceasing thirst, urgent breathing with teeth grinding and thirst, and increased thirst after drinking water—these nine symptoms are especially not Excess Heat. One should urgently warm the interior and rescue collapse, nourishing Body Fluids to prevent death from itching/collapse, wheezing, and thirst. If treated as a Heat syndrome, danger and death will appear immediately.
Thirst at the onset of the disease often has corresponding patterns, but one must distinguish between responding (normal) and non-responding (abnormal) signs. In the first two to three days, fever and thirst indicate toxin accumulation in the interior and Heat pathogen scorching; treatment should avoid premature pushing-out of toxins. After the fourth day, fever and thirst indicate Body Fluids leaking outward to transform into pus; treatment should use Ren Shen (Ginseng); these are all responding signs. Only after crusting, when pathogens are fully transformed and no residual evil remains in the interior, if extreme thirst suddenly appears, it indicates gradual consumption of True Qi and rising Fire-Toxin; this is a non-responding sign. Urgently provide detoxification, nourishing Yin, generating fluids, and benefiting the throat. If thirst gradually decreases, it is auspicious; if it turns to increased thirst, it will inevitably change into wheezing and distension, becoming critical.
Those who can eat but have thirst have Lung Heat. The *Classic* says: Heart transfers Heat to the Lung, transmitting as Ge Xiao (Diaphragmatic Consumption). This is due to Heart Fire flaming upward, assaulting Lung Metal, steaming the diaphragm, and consuming Body Fluids. The pathology is in the Upper Jiao, treated with Renshen Baihu Tang (Ginseng White Tiger Decoction) added with Huanglian (Coptis). Those who cannot eat but have thirst have Spleen Deficiency. Because the Spleen's Yuan Qi is weak, it cannot transport Body Fluids for the Stomach. The pathology is in the Middle Jiao, and one must guard against diarrhea; thus, Shenyin Baizhu San (Ginseng and Atractylodes Macrocephala Powder) is appropriate. Spontaneous diarrhea with thirst indicates the pathogen has transmitted to the Kidney. Spontaneous diarrhea with thirst belongs to Foot Shaoyin (Kidney) Deficiency, hence the desire for water to self-rescue. The Kidney governs the five fluids, its meridians connect to the Lung, and it ties to the root of the tongue. If the pathogen transmits to the Kidney, opening and closing functions fail, causing spontaneous diarrhea; diarrhea causes Body Fluids to flow downward, drying up Kidney Water, which cannot moisten the tongue upward, causing extreme thirst. The pathology is in the Lower Jiao, treated with Yigong San (Extraordinary Merit Powder) to warm.