寒热者，谓往来寒热也。经曰、血弱气虚，腠理受邪，阴阳相胜，邪正相争而作矣。若邪气之入也，而正
气不与之争，但热而无寒也。今热而寒者，谓正气与邪气分争，于是寒热作矣。争则气郁不发于外，
而寒热争焉，争甚则愤然而热，故寒已而外热作焉，斯乃寒热之理也。或谓寒热者，阴阳争胜也，以
阳胜则热，阴胜则寒，斯乃阴阳之争也。何则。以寒为阴而热为阳，以里为阴而表为阳。邪之客于表
者为寒，邪与阳争，则为寒矣。邪之入于里者为热，邪与阴争，则为热矣。其邪半在表半在里，外与阳
争而为寒，内与阴争而为热，是则表里不拘，内外不定，或出或入，由是寒热且往且来而间作也，是
以往来南戴热，多属半表半里之症。一云、阳不足，则阴邪出表而与之争，故阴胜而为寒，阴不足，则阳
邪入里而与之争故阳胜而为热。邪居表多则多寒，邪居里多则多热，邪在半表半里，则寒热平半
矣。审其寒热多少，视其邪气深浅，俱用小柴胡汤和之，若寒多、加桂枝，热多、加大黄，是大法也。
似疟者，与发热恶寒，往来寒热，相似而实非也。寒热似疟者，作止有时，若往来寒热，则作止无时，或
往或来，日有至三五发，甚则十数发势是也。至于发热恶寒者，为发热时恶寒并不见，恶寒时发热
亦不见也。不若此寒已而热，热已而寒也。然有太阳似疟，有阳明似疟，有厥阴似疟，有妇人热入血
室似疟，症虽同而用药异也。
寒热病，指的是寒热往来的症状。《内经》说：血弱气虚，腠理（皮肤毛孔及肌肉间隙）受邪侵袭，阴阳相互偏胜，邪气与正气相争而发病。如果邪气侵入，而正气不与它抗争，就只有发热而没有恶寒。现在既有发热又有恶寒，是因为正气与邪气分别抗争，于是产生了寒热交替的症状。抗争时，气机郁结不能发散于体表，所以出现寒热交争；抗争剧烈则愤激而发热，因此恶寒过后体表发热发作，这就是寒热往来的机理。有人说寒热往来是阴阳争胜的结果，因为阳胜则热，阴胜则寒，这是阴阳之间的争斗。为什么呢？以寒为阴、热为阳，以里为阴、表为阳。邪气客居在体表表现为恶寒，邪气与阳气相争，就表现为恶寒；邪气深入体内表现为发热，邪气与阴气相争，就表现为发热。如果邪气一半在表、一半在里，向外与阳气相争就产生恶寒，向内与阴气相争就产生发热，这样表里的界限就不固定，内外的位置也不确定，邪气或出或入，因此寒热往来且间歇性发作，所以寒热往来多属于半表半里的证候。另一种说法认为：阳气不足，阴邪外出到体表与之相争，所以阴胜而表现为恶寒；阴气不足，阳邪深入体内与之相争，所以阳胜而表现为发热。邪气多在体表则恶寒多，邪气多在体内则发热多，邪气在半表半里，则寒热程度相当。仔细审察寒热的多少，观察邪气的深浅，都使用小柴胡汤来和解；如果恶寒多，加桂枝；如果发热多，加大黄，这是基本法则。
所谓“似疟”，是指症状与发热恶寒、寒热往来相似，但实际上并非如此。寒热似疟的症状，发作和停止都有定时；而真正的寒热往来，发作和停止没有定时，有时发作有时停止，一天可能发作三到五次，严重的甚至十几次，气势猛烈。至于发热恶寒并见，是指在发热的时候并不见恶寒，在恶寒的时候也不见发热，不像先恶寒后发热、先发热后恶寒那样交替出现。然而，有太阳病似疟，有阳明病似疟，有厥阴病似疟，有妇人热入血室似疟，虽然症状相同，但用药不同。
The condition of *Hanre* (alternating chills and fever) refers to the symptom of alternating chills and fever. The Classics state: "When blood is deficient and Qi is weak, the Couli (pores and interstices) are invaded by pathogenic factors; Yin and Yang contend with each other, and the struggle between pathogenic factors and healthy Qi gives rise to the disease." If pathogenic factors invade but healthy Qi does not contend with them, there will be only fever without chills. Now, when both fever and chills occur, it is because healthy Qi and pathogenic factors are contending separately, thus generating alternating chills and fever. During this contention, Qi becomes stagnant and fails to disperse outward, leading to a struggle of chills and fever. When the contention is severe, it becomes intense and results in fever; therefore, after the chills subside, external heat manifests. This is the mechanism of alternating chills and fever. Some argue that alternating chills and fever result from the contention between Yin and Yang, as excess Yang leads to heat and excess Yin leads to cold; this is the contention of Yin and Yang. Why? Because cold is Yin and heat is Yang, while the interior is Yin and the exterior is Yang. When pathogenic factors lodge in the exterior, they manifest as chills; when pathogens contend with Yang, chills occur. When pathogenic factors enter the interior, they manifest as heat; when pathogens contend with Yin, heat occurs. If the pathogen is half in the exterior and half in the interior, contending outwardly with Yang produces chills, and contending inwardly with Yin produces heat. Thus, the boundaries of exterior and interior are not fixed, and the location of inside and outside is unstable; the pathogen moves in and out, causing alternating chills and fever to occur intermittently. Therefore, alternating chills and fever mostly belong to the Shaoyang syndrome (half-exterior, half-interior). Another view holds: if Yang is insufficient, Yin pathogenic factors emerge to the exterior and contend with it, resulting in excess Yin and thus chills; if Yin is insufficient, Yang pathogenic factors enter the interior and contend with it, resulting in excess Yang and thus fever. If pathogens reside mostly in the exterior, chills predominate; if pathogens reside mostly in the interior, heat predominates; if pathogens are in the half-exterior/half-interior region, chills and heat are balanced. By carefully examining the degree of chills and heat, and observing the depth of the pathogenic factors, Xiaochaihu Tang (Minor Bupleurum Decoction) is used to harmonize. If chills are predominant, add Guizhi (Cinnamon Twig); if heat is predominant, add Dahuang (Rhubarb Root); this is the fundamental principle.
"Si Nue" (malaria-like) refers to symptoms that resemble fever with aversion to cold and alternating chills and fever, but are essentially different. In malaria-like chills and fever, the onset and cessation have a regular timing; whereas in true alternating chills and fever, the onset and cessation are irregular. It may occur or recur, happening three to five times a day, and in severe cases, more than ten times, with intense manifestations. As for simultaneous fever and aversion to cold, it means that during fever, aversion to cold is not present, and during aversion to cold, fever is not present, unlike the alternation of chills followed by heat, or heat followed by chills. However, there are Taiyang syndrome resembling malaria, Yangming syndrome resembling malaria, Jueyin syndrome resembling malaria, and women's syndrome of heat entering the Blood Chamber resembling malaria. Although the symptoms are similar, the medicinal prescriptions differ.