阴阳大论云春气温和夏气暑热秋气清凉冬气冷冽此则四时正气之序也冬时严寒万类深藏
君子固密则不伤于寒触冒之者乃名伤寒耳其伤于四时之气皆能为病以伤为毒者以气而成杀厉
之患也中而即病者名曰伤寒不即病者寒毒藏于肌肤至春变为温病至夏变为暑病暑病者热极重
于温也是以辛苦之人春夏必多温热病皆由冬时触寒所致非时行之气也凡时行者春时应暖而反
寒夏时应大热而反大凉秋时应凉而反大热冬时应大寒而反温此非其时而有其气是以一岁之中
长幼之病多相似者此则时行疫厉之气也夫欲究知四时之气为病及时行疫气之法皆当按斗历以
占之九月霜降节后宜渐寒而冬大寒至正月雨水节后宜解也所以谓之雨水者以冰雪解而为雨水
故也至惊蛰节后气渐和暖向夏大热至秋便凉从霜降以后至春分以前凡有触冒霜露体中寒即病
者谓之伤寒也如冬有非节之暖者名曰冬温冬温之毒与伤寒大异冬温复有先后更相重沓亦有轻
重为治不同症如后章从立春节后其中无暴大寒又不冰雪而有人壮热为病者此属春时阳气发于
冬时伏寒变为温病从春分以后至秋分节前天有暴寒者皆为时行寒疫也三月四月或有暴寒其时
阳气尚弱为寒所折病热犹轻五月六月阳气已盛为寒所折病热则重七月八月阳气已衰为寒所折
病热亦微其寒病与温及暑病相似但治有殊耳
《阴阳大论》说：春季的气候温和，夏季的气候暑热，秋季的气候清凉，冬季的气候冷冽，这是四季正常气候的次序。
到了冬天，天气极其严寒，万物都深藏起来。
君子（指懂得养生的人）如果注意固密保暖，就不会被寒邪所伤；如果触犯冒犯寒邪，这才叫做“伤寒”。
凡是受到四时不正之气侵袭都能致病，但把“伤”视为毒害，是因为这种气能造成杀厉般的祸患。
如果在受邪后立即发病的，叫做“伤寒”；如果不立即发病，寒毒潜伏在肌肤之间，到了春天就转变为温病，到了夏天就转变为暑病。
因此，辛勤劳作的人在春夏季节必定多患温热病，这都是因为冬天时触犯了寒邪所致，并非属于时行疫气。
凡是时行疫气，是指春天应该温暖却反而寒冷，夏天应该大热却反而大凉，秋天应该凉爽却反而炎热，冬天应该大寒却反而温暖。
这种在非其时的季节出现了相应的气候异常，所以一年之中，无论老少患病症状大多相似，这就是所谓的时行疫厉之气。
想要探究四时之气致病以及时行疫气的规律方法，都应该依据斗历（古代观测天象以定节气的历法）来占测推断。
九月霜降节气过后，气候应当逐渐变寒，直到冬至大寒时节达到极寒。
正月雨水节气过后，气候应当开始解冻回暖，之所以称为“雨水”，是因为冰雪融化变成了雨水。
到了惊蛰节气后，气候逐渐和暖，接近夏天时变得大热，到了秋天便转凉。
从霜降以后到春分以前，凡是有触犯冒犯霜露，体内感受寒邪而立即发病的，叫做伤寒。
如果冬天出现不符合节令的温暖气候，叫做“冬温”。
冬温又有先后相继、病情重叠的情况，也有轻重之分，治疗方法也不同，症状详见后章。
从立春节后，其中如果没有突然的大寒，也没有冰雪，但有人出现壮热病症的，这属于春季阳气发动，将冬季潜伏的寒邪转化为温病。
从春分以后到秋分节气前，天空出现突然的寒凉气候，都属于时行的寒疫。
The *Yin Yang Da Lun* (Great Treatise on Yin and Yang) states: The Qi of spring is mild and harmonious, the Qi of summer is hot and sultry, the Qi of autumn is cool and clear, and the Qi of winter is cold and biting. This constitutes the normal sequence of the four seasons' Qi.
When winter arrives, the cold is extremely severe, and all things hide deeply.
A gentleman (one who understands health preservation), if he maintains solidity and warmth, will not be injured by cold; however, if he exposes himself to and offends against the cold, this is termed "Shanghan" (Cold Damage).
Any invasion by the Qi of the four seasons that deviates from the norm can cause disease. Regarding "injury" as toxic, it is because such Qi can bring about calamitous and lethal disasters.
If illness occurs immediately upon exposure, it is named "Shanghan" (Cold Damage); if illness does not occur immediately, the Cold Toxicity hides within the skin and muscles, transforming into "Wenbing" (Warm Disease) in spring, and into "Shubing" (Summerheat Disease) in summer. Shubing (Summerheat Disease) involves heat that is extremely severe, heavier than that of Wenbing (Warm Disease).
Therefore, people engaged in hard labor must frequently suffer from Warm-Heat diseases during spring and summer. These are all caused by offending cold during winter, and do not belong to the category of "Shixing Yiqi" (Seasonal Epidemic Qi).
Whenever there is "Shixing" (Epidemic occurrence), it refers to situations where spring should be warm but is instead cold, summer should be intensely hot but is instead very cool, autumn should be cool but is instead hot, and winter should be intensely cold but is instead warm. Because the climate appears contrary to the season, throughout the year, the illnesses affecting both young and old are often similar. This is what is called the Qi of seasonal epidemic pestilence.
To thoroughly understand the diseases caused by the Qi of the four seasons and the methods regarding seasonal epidemics, one must consult the Douli (ancient astronomical calendar used to determine solar terms) for divination and inference.
After the Frost Descent (Shuangjiang) solar term in the ninth month, the weather should gradually become cold until the Great Cold (Dahan) of winter arrives. After the Rain Water (Yushui) solar term in the first month, the weather should begin to thaw. It is called "Rain Water" because ice and snow melt to become rainwater.
After the Awakening of Insects (Jingzhe) solar term, the Qi gradually becomes mild and warm, becoming intensely hot towards summer, and turning cool in autumn. From after Frost Descent until before the Spring Equinox (Chunfen), anyone who offends and exposes themselves to frost and dew, causing internal cold and immediate illness, is suffering from Shanghan (Cold Damage).
If there is unseasonable warmth during winter, it is named "Dongwen" (Winter Warmth). The toxicity of Dongwen (Winter Warmth) is vastly different from Shanghan (Cold Damage). Dongwen (Winter Warmth) may also have sequential occurrences with overlapping symptoms, varying in severity, and requiring different treatments; symptoms are detailed in the following chapter.
After the Beginning of Spring (Lichun) solar term, if there is no sudden severe cold and no ice or snow, yet some individuals suffer from high fever, this belongs to the transformation of latent winter cold into Wenbing (Warm Disease) due to the movement of spring Yang Qi.
From after the Spring Equinox (Chunfen) until the Autumn Equinox (Qiufen) solar term, any sudden cold weather in the sky constitutes a seasonal Cold Epidemic (Han Yi).
In the third and fourth months, there may be sudden cold. At this time, Yang Qi is still weak; being restrained by cold, the resulting fever is relatively mild. In the fifth and sixth months, Yang Qi is already abundant; being restrained by cold, the resulting fever is heavy. In the seventh and eighth months, Yang Qi is declining; being restrained by cold, the resulting fever is also slight.
These Cold diseases resemble Warm diseases and Summerheat diseases in symptoms, but their treatment methods differ significantly.