病疫之由，昔以为非其时有其气，春应温而反大寒，夏应热而反大凉，秋应凉而
反大热，冬应
因风雨阴晴，稍为损益，假令秋热必多晴，春寒因多雨，较之亦天地之常事，未必多疫也。
伤寒与中暑，感天地之常气，疫者感天地之疠气，在岁有多寡；在方隅有浓薄；在四时有
盛衰。此气之来，无论老少强弱，触之者即病。邪自口鼻而入，则其所客，内不在脏腑，外不在
经络，舍于伏脊之内，去表不远，附近于胃，乃表里之分界，是为半表半里，即《针经》所
谓横连膜原是也。胃为十二经之海，十二经皆都会于胃，故胃气能敷布于十二经中，而荣养百骸
，毫发之间，弥所不贯。凡邪在经为表，在胃为里，今邪在膜原者，正当经胃交关之所，故
为
如折；如浮越于阳明，则有目痛、眉棱骨痛、鼻干；如浮越于少阳，则有胁痛、耳聋、寒热
、呕而口苦。大概述之，邪越太阳居多，阳明次之，少阳又其次也。邪之所着，有天受，有
传染，所感虽殊，其病则一。凡人口鼻之气，通乎天气，本气充满，邪不易入，本气适逢亏
欠，呼吸之间，外邪因而乘之。昔有三人，冒雾早行，空腹者死，饮酒者病，饱食者不病。疫
邪所着，又何异耶？若其年气来盛厉，不论强弱，正气稍衰者，触之即病，则又不拘于此矣
。其感之深者，中而即发；感之浅者，邪不胜正，未能顿发，或遇饥饱劳碌，忧思气怒，正
气被伤，邪气始得张溢，营卫营运之机，乃为之阻，吾身之阳气，因而屈曲，故为病热。其始
也，格阳于内，不及于表，故先凛凛恶寒，甚则四肢厥逆。阳气渐积，郁极而通，则厥回而
中
外皆热。至是但热而不恶寒者，因其阳气之周也。此际应有汗，或反无汗者，存乎邪结之轻
重
也。即便有汗，乃肌表之汗。若外感在经之邪，一汗而解。今邪在半表半里，表虽有汗，
徒
损真气，邪气深伏，何能得解？必俟其伏邪渐退，表气潜行于内，乃作大战，精气自内由膜
中以达表，振战止而复热，此时表里相通，故大汗淋漓，衣被湿透，邪从汗解，此名战汗。
当即脉静身凉，神清气爽，划然而愈。然有自汗而解者，但出表为顺，即不药亦自愈也。伏
邪未退，所有之汗，止得卫气渐通，热亦暂减，超时复热。午后潮热者，至是郁甚，阳气与
时消息也，自后加热而不恶寒者，阳气之积也。其恶寒或微或甚，因其人之阳气盛衰也；其
发热或久或不久，或昼夜纯热，或黎明稍减，因其感邪之轻重也。疫邪与疟仿佛，但疟不传胃
，惟疫乃传胃。始则皆先凛凛恶寒，既而发热，又非若伤寒发热而兼恶寒也。至于伏邪动作
，方有变证其变或从外解，或从内陷。从外解者顺，从内陷者逆。更有表里先后不同∶有
先表而后里者，有先里而后表者，有但表而不里者，有但里而不表者，有表里偏胜者，有表
里分传者，有表而再表者，有里而再里者。有表里分传而又分传者。从外解者，或发斑，或战汗、狂汗、自汗、盗汗
；从内陷者，胸膈痞闷，心下胀满，或腹中痛，或燥结便秘，或热结旁流，或协热下利，或
呕吐、恶心、谵语、舌黄、舌黑、苔刺等证。因证而知变，因变而知治。此言其大略，详见
脉证治法诸条。
瘟疫的成因，过去人们认为是因为非其时而有其气，即春天应当温暖却反而大寒，夏天应当炎热却反而大凉，秋天应当凉爽却反而大热，冬天应当寒冷却反而大温。这是因为风雨阴晴的变化，气候稍有增减，例如秋天炎热必然多晴天，春天寒冷必然多雨水，这相比起来也是天地间的常事，未必会导致多发的瘟疫。
伤寒和中暑，是感受天地间的正常气候之气；而瘟疫，则是感受天地间的疠气（疫毒之气）。这种疠气在一年中有多少之分，在不同地域有浓薄之别，在四季中有盛衰之异。这种邪气来袭时，无论老少强弱，一旦接触就会发病。邪气从口鼻侵入，其所停留之处，内不在脏腑，外不在经络，而是潜伏于脊骨内部的伏脊之内，距离体表不远，靠近胃部，这是表与里的分界，称为“半表半里”，也就是《针经》中所说的“横连膜原”。
胃为十二经脉之海，十二经脉都汇聚于胃，所以胃气能够布散到十二经脉中，滋养全身百骸，毫发之间，无不贯通。凡是邪气在经脉中属于表证，在胃中属于里证。现在邪气停留在膜原，正处于经脉与胃交关的位置，所以被称为半表半里。如果邪气浮越于阳明经，就会出现眼睛疼痛、眉棱骨痛、鼻子干燥；如果浮越于少阳经，就会出现胁肋疼痛、耳聋、忽冷忽热、呕吐且口苦。大概来说，邪气侵犯太阳经的最多，阳明经次之，少阳经又其次。
邪气之所以附着于人，有天受（通过空气传播）和传染（直接接触传播）两种方式，虽然感受途径不同，但致病机理是一样的。人的口鼻之气与天气相通，如果人体本气充足，邪气不易侵入；如果人体本气恰好亏欠，在呼吸之间，外邪就趁机乘虚而入。
过去有三个人，冒着雾气早起赶路，空腹的人死了，喝酒的人病了，吃饱饭的人没病。疫邪附着于人，又何尝不是如此呢？如果那年疠气来势凶猛强烈，不论体质强弱，只要正气稍显衰弱，接触就会发病，这又不拘泥于上述饮食状况了。
感邪深重的，中邪后立即发病；感邪浅薄的，邪气不胜正气，不能立即发作，或者遇到饥饱劳碌、忧思气怒等情况，导致正气受伤，邪气才得以张溢蔓延，营卫运行的机制因此受阻，我身的阳气因而屈曲不畅，所以表现为发热。起初，阳气被格拒于内，不能达于体表，所以先感到凛凛恶寒，严重时四肢厥逆。随着阳气逐渐积聚，郁结至极而通达，则厥逆回返，内外皆热。到了这个时候，只有发热而没有恶寒，是因为阳气已经周流全身。此时应当有汗，或者反而无汗，这取决于邪气结聚的轻重程度。
即便有汗，也只是肌表的汗。如果是外感邪气在经脉，一出汗就能痊愈。现在邪气在半表半里，虽然体表有汗，徒然损耗真气，邪气深伏，怎么能解除呢？必须等待潜伏的邪气逐渐消退，表气潜行于内，于是发生大战，精气从内部由膜原到达体表，出现振颤战栗后停止，随后再次发热，此时表里相通，所以大汗淋漓，衣被湿透，邪气随汗而解，这叫做“战汗”。战汗之后，应当脉象平静、身体凉爽、神清气爽，豁然痊愈。然而也有自汗而解的情况，只要汗出表解为顺证，即使不吃药也能自愈。
如果伏邪未退，所出的汗，只能使卫气逐渐通畅，热势暂时减轻，超时后热度复升。午后潮热的，是因为此时郁结甚重，阳气与时令消长有关。此后热度加重而不恶寒，是阳气积聚的结果。其恶寒或轻或重，取决于人的阳气盛衰；其发热或久或不久，或昼夜纯热，或黎明稍减，取决于感邪的轻重。
疫邪与疟疾相似，但疟疾不传胃，只有瘟疫会传胃。开始时都是先感到凛凛恶寒，继而发热，这又不是像伤寒那样发热兼有恶寒。至于伏邪发动，才会有变证。其变化有的从外表解，有的从内陷。从外表解为顺证，从内陷为逆证。更有表里先后顺序的不同：有先治表而后治里的，有先治里而后治表的，有只治表不治里的，有只治里不治表的，有表里偏胜的，有表里分传的，有表证再发为表证的，有里证再发为里证的，有表里分传而又进一步分传的。
从外表解的，或发斑疹，或战汗、狂汗、自汗、盗汗；从内陷的，出现胸膈痞闷，心下胀满，或腹中疼痛，或燥结便秘，或热结旁流，或协热下利，或呕吐、恶心、谵语、舌苔黄、舌苔黑、舌生芒刺等证候。根据证候来了解病情变化，根据病情变化来确定治疗方法。这里只是概述大概，详细内容详见脉证治法诸条。
The cause of plague (Yi, epidemic disease) was formerly believed to be due to "Qi present when it should not be," meaning spring should be warm but is instead severely cold, summer should be hot but is instead severely cool, autumn should be cool but is instead severely hot, and winter should be cold but is instead severely warm. This is because changes in wind, rain, yin, and yang cause slight损益 (increase or decrease) in climate; for instance, autumn heat implies more sunny days, and spring cold implies more rainy days. Compared to this, these are normal phenomena of Heaven and Earth and do not necessarily lead to frequent epidemics.
Shanghan (Cold Damage) and Zhongshu (Heat Stroke) result from contracting the normal Qi of Heaven and Earth. Plague, however, results from contracting Liqi (pestilential Qi). The abundance or scarcity of this Qi varies by year, its density or thinness varies by region, and its flourishing or declining state varies by season. When this Qi arrives, regardless of age or strength, anyone who contacts it will fall ill. The pathogen enters via the mouth and nose. Its residence is neither inside the Zang-Fu organs nor outside in the meridians, but rather lodges within the Fuji (hidden spine), close to the surface but near the Stomach. This is the boundary between exterior and interior, known as Banbiao Banli (Half-Exterior Half-Interior), which corresponds to what the *Zhenjing* (Classic of Acupuncture) calls "Henglian Moyuan" (transversely connecting the Membrane Source).
The Stomach is the Sea of the Twelve Meridians, and all Twelve Meridians converge at the Stomach. Therefore, Stomach Qi can distribute throughout the Twelve Meridians, nourishing the entire body, penetrating every hair and fiber. Generally, pathogens in the meridians constitute an Exterior syndrome, while those in the Stomach constitute an Interior syndrome. Now, since the pathogen resides in the Moyuan (Membrane Source), it is exactly at the junction where the meridians and the Stomach intersect; thus, it is called Half-Exterior Half-Interior. If the pathogen floats to the Yangming Meridian, there will be eye pain, pain in the supraorbital ridge, and nasal dryness. If it floats to the Shaoyang Meridian, there will be hypochondriac pain, deafness, alternating chills and fever, vomiting, and a bitter taste in the mouth. Broadly speaking, invasion of the Taiyang Meridian is most common, followed by Yangming, and then Shaoyang.
Pathogens attach to humans through Tian Shou (airborne transmission) and Chuan Ran (direct contact transmission). Although the mode of infection differs, the pathology is the same. The Qi of human mouth and nose connects with the Qi of Heaven. If one's Ben Qi (root Qi) is abundant, pathogens find it difficult to enter. If Ben Qi is deficient, external pathogens take advantage during respiration. In the past, three people traveled early in the morning through fog: the empty-stomached person died, the person who drank alcohol became ill, and the well-fed person remained healthy. Is it not similar for how plague pathogens attach? However, if the annual Qi is extremely fierce, regardless of constitution, anyone whose Zheng Qi (Upright Qi) is slightly declined will fall ill upon contact, disregarding the aforementioned dietary conditions.
Deep infection leads to immediate onset. Shallow infection means the pathogen cannot overcome the Zheng Qi, so it does not manifest immediately. It may only erupt after factors like hunger, fullness, fatigue, worry, anger, etc., injure the Zheng Qi, allowing the pathogen to spread. The mechanism of Ying and Wei (Nutritive and Defensive Qi) circulation becomes blocked, causing the body's Yang Qi to become constrained, resulting in fever. Initially, Yang Qi is rejected internally and cannot reach the exterior, leading first to severe aversion to cold, and in severe cases, cold extremities (Jue Ni). As Yang Qi gradually accumulates, depression reaches its peak and then opens up, reversing the cold extremities and causing heat throughout the interior and exterior. At this stage, there is only fever without aversion to cold because Yang Qi has circulated fully. Sweat should occur here; if absent, it depends on the severity of pathogen binding. Even if sweat occurs, it is merely superficial. For exterior pathogens in the meridians, one sweating episode resolves the condition. But now that the pathogen is in the Half-Exterior Half-Interior, although there is sweat on the surface, it vainly depletes True Qi, and the deeply lodged pathogen cannot be resolved. One must wait for the latent pathogen to recede gradually, allowing Surface Qi to move inward, triggering a major battle. Jing Qi (Essence Qi) moves from the interior to the exterior via the Moyuan. After shaking and trembling cease, fever returns. At this moment, interior and exterior connect, leading to profuse sweating, soaking clothes and bedding, and the pathogen resolving through sweat. This is called Zhan Han (Battle Sweating). Immediately afterward, the pulse should calm, the body cool, and the spirit clear, resulting in sudden recovery. There are also cases of resolution through spontaneous sweating; as long as the exterior releases, it is a favorable progression, and self-healing occurs even without medicine.
If the latent pathogen has not receded, the sweat produced only allows Wei Qi to gradually circulate, temporarily reducing heat, which rebounds after time passes. Tidal fever in the afternoon indicates severe depression at that time, related to the waxing and waning of Yang Qi with the seasons. Subsequent intensification of heat without aversion to cold is due to Yang Qi accumulation. Mild or severe aversion to cold depends on the strength or weakness of the person's Yang Qi. Duration of fever, whether continuous day and night or slightly reduced at dawn, depends on the severity of the contracted pathogen.
Plague pathogens resemble Nuojing (Malaria) pathogens, but Malaria does not transmit to the Stomach, whereas Plague does. Both begin with severe aversion to cold followed by fever, unlike Shanghan where fever accompanies aversion to cold. Once the latent pathogen activates, transformation syndromes appear. Transformations may resolve from the exterior or sink into the interior. Resolution from the exterior is favorable; sinking into the interior is unfavorable. Furthermore, the sequence of exterior and interior involvement varies: some involve exterior first then interior, others interior first then exterior, some only exterior without interior, some only interior without exterior, some show predominance of either exterior or interior, some transmit simultaneously to both, some recur in the exterior, some recur in the interior, and some transmit to both and then further subdivide.
Resolution from the exterior manifests as macules (Ban), Battle Sweating, Manic Sweating, Spontaneous Sweating, or Night Sweats. Sinking into the interior manifests as chest and diaphragm oppression, epigastric distension, abdominal pain, dry constipation, heat-binding with purging of watery stool, diarrhea with heat, vomiting, nausea, delirious speech, yellow tongue coating, black tongue coating, or prickles on the tongue. By understanding the symptoms, one knows the transformation; by knowing the transformation, one determines the treatment. This is a general overview; detailed discussions are found in the sections on Pulse, Symptoms, and Treatment Methods.