夫伤寒者，冬时天气严寒，水冰地冻，而成杀厉之气，正乃肾与膀胱坎水用事，体虚触冒之人，
中而即病，曰伤寒；不即病者，乃寒毒藏于肌肤。此因肾水涸竭，春木无以发生，热不能发泄，藏
郁于内，遇感而发，至春变为温病，至夏变为热病，热病重于温病也。虽曰伤寒，实为热病。非时
行之气，春应暖而反寒，夏应热而反凉，秋应凉而反热，冬应寒而反温，此非其时而有其气，是一
岁之中，长幼病皆相似者也。是时行不正之气，非暴厉之气。暴厉者，疫病也。疫病者，乃春分至
秋分前，天有暴寒，皆为时行之寒疫也。又有四时之正气者，春气温和，夏气暑热，秋
气清凉，冬气凛冽，此四时之正气也。然正气亦能为病，春伤于风，夏必飧泄；夏伤于暑，秋必疟
痢；秋伤于湿，冬必咳嗽；冬伤于寒，春必温病，总曰伤寒。病自外而入，或入于阳，或入于阴，
非但始于太阳，终于厥阴。或有太阳伤者，或有传至一二经而止者，或有始终只在一经者，或有越
经而传者，或有入太阳不作郁热，便入少阴而成阴证者，或有直中阴经而成寒证者，缘经无明文，
故多妄治。若三阳传至三阴之阴证，外虽有厥冷，内有热邪耳。若不发热，四肢便厥，恶寒者，此
是直中阴经之阴证也。盖先起三阳气分，传入三阴血分，则热深厥亦深矣。此亢则害，承乃制者欤，
热极反兼寒化也。先热后厥者，传经阴证也，故宜四逆散、大承气，看微甚而下之。如初病无热便
厥者，此直中阴经之寒证也，宜四逆汤温之。
所谓伤寒，是指在冬季天气严寒、水结冰地冻时，形成一种具有杀伤力的厉气。此时正是肾与膀胱（属坎水）当令的季节，体质虚弱的人如果触犯冒犯了这种寒气，随即发病的，称为伤寒；如果不立即发病，寒毒就会潜伏在肌肤之间。这是因为肾水枯竭，导致春季木气无法生发，热气不能发泄，郁结于体内，遇到感应而发作，到了春天就变成温病，到了夏天就变成热病，其中热病的病情比温病更重。虽然名为伤寒，实际上属于热病。
如果不是应时的气候之气，而是春天本该温暖却反而寒冷，夏天本该炎热却反而凉爽，秋天本该凉爽却反而炎热，冬天本该寒冷却反而温暖，这种非其时而出现的气候，会导致一年之中无论老少，患病症状都相似。这是指时行不正之气，而不是暴烈的厉气。所谓的暴烈之气，就是疫病。疫病是指从春分到秋分之前，天空出现突然的寒冷，都属于时行的寒疫。
另外还有四时的正常气候之气：春天的气候温和，夏天的气候暑热，秋天的气候清凉，冬天的气候凛冽，这就是四时的正气。然而正气也能致病：春天伤于风邪，夏天必定发生完谷不化的泄泻；夏天伤于暑邪，秋天必定发生疟疾和痢疾；秋天伤于湿邪，冬天必定发生咳嗽；冬天伤于寒邪，春天必定发生温病，统称为伤寒。
疾病从外部侵入人体，或者进入阳经，或者进入阴经，并不只是始于太阳经，终于厥阴经。有的仅伤及太阳经，有的传变到一或二经就停止，有的始终只在一经，有的跨越经络传变，有的进入太阳经后没有形成郁热，就直接进入少阴经而成为阴证，有的直接中于阴经而成为寒证。由于经典中没有明确的条文记载，所以医生们常常错误治疗。如果是三阳经传变至三阴经形成的阴证，虽然外部有四肢厥冷，但内部仍有热邪。如果不发热，四肢立刻厥冷，且恶寒的，这是直接中于阴经的阴证。
大概是因为先起于三阳的气分，传入三阴的血分，那么热越深，厥冷也就越深。这难道是“亢盛则有害，承制才能平衡”的道理吗？这是热极反而兼有寒化的表现。先发热后出现厥冷的，是传经形成的阴证，所以适宜用四逆散或大承气汤，根据病情的轻重来攻下。如果初病就没有发热而直接厥冷的，这是直接中于阴经的寒证，适宜用四逆汤来温补。
The so-called Shanghan (Cold Damage) refers to the occurrence of a lethal pathogenic factor during winter when the weather is severely cold, water freezes, and the ground hardens. At this time, the Kidney and Bladder (belonging to Kan Water) are in their dominant phase. Individuals with deficient constitutions who contract this cold qi may fall ill immediately, which is termed Shanghan; if they do not fall ill immediately, the cold toxin hides within the skin and muscles. This occurs because the Kidney Water is depleted, preventing the Spring Wood Qi from generating, causing heat to fail in dissipating and stagnating internally. Upon encountering a trigger, it manifests: by Spring it transforms into Wenbing (Warm Disease), and by Summer it transforms into Rebing (Heat Disease), with Rebing being more severe than Wenbing. Although named Shanghan, it is essentially a Heat Disease.
If the climate does not follow the proper seasonal order—spring should be warm but turns cold, summer should be hot but turns cool, autumn should be cool but turns hot, and winter should be cold but turns warm—this abnormal climate appearing out of season causes people of all ages throughout the year to suffer from similar illnesses. This refers to the irregular Qi of Shixing (Seasonal Epidemic), rather than a violent lethal Qi. The violent lethal Qi refers to Yi Bing (Epidemic Disease). Yi Bing refers to sudden cold occurring between the Spring Equinox and before the Autumn Equinox, which constitutes a seasonal Cold Epidemic.
There are also the normal Qi of the four seasons: Spring Qi is mild and harmonious, Summer Qi is hot and sultry, Autumn Qi is clear and cool, and Winter Qi is biting and cold; these constitute the Normal Qi of the four seasons. However, Normal Qi can also cause disease: injury by Wind in Spring leads to undigested diarrhea (Sunxie) in Summer; injury by Summerheat in Summer leads to Malaria and Dysentery (Nuli) in Autumn; injury by Dampness in Autumn leads to Cough in Winter; injury by Cold in Winter leads to Warm Disease in Spring. All are collectively termed Shanghan.
Diseases enter from the exterior, either entering the Yang meridians or the Yin meridians. They do not necessarily start at the Taiyang (Greater Yang) and end at the Jueyin (Terminal Yin). Some injuries remain only in Taiyang; some transmit to one or two meridians and stop; some remain in a single meridian throughout; some transmit across meridians; some enter Taiyang without forming stagnant heat, then directly enter Shaoyin (Lesser Yin) becoming a Yin syndrome; some directly strike the Yin meridians becoming a Cold syndrome. Due to the lack of explicit text in classics, many physicians treat them erroneously. If Yang meridians transmit to Yin meridians forming a Yin syndrome, although there is external cold extremities (Jue Leng), there is internal Heat pathogen. If there is no fever, limbs become cold immediately, and there is aversion to cold, this is a Yin syndrome caused by direct strike to the Yin meridians. Generally, starting in the Qi aspect of the Three Yang meridians and transmitting to the Blood aspect of the Three Yin meridians results in deeper heat leading to deeper cold extremities. Is this not the principle that "excess brings harm, and control restores balance"? It is Heat extreme manifesting as Cold transformation. Fever followed by cold extremities indicates a Yin syndrome transmitted via meridians, for which Sini San (Frigid Extremities Powder) or Dachengqi Tang (Major Order the Qi Decoction) should be used to purge according to severity. If cold extremities occur initially without fever, this is a Cold syndrome from direct strike to the Yin meridians, for which Sini Tang (Frigid Extremities Decoction) should be used to warm.