凡治伤寒、瘟疫，医者最重初次得疾，至于抄复，谓死者盖寡，每视为最轻而
谩不经意焉。盖谓抄复之病，人身之经络、脏腑皆前次瘟邪所曾经传遍之所，则此番不过由熟路
而行，故邪气易出也。古人原有此论，岂知此第语其常也。独瘟疫盛行之时则不然，盖是时疫气
所积者浓，即无气食劳损之因，尚有重感疠气而复者，更有前番余邪稍有未净，再酝酿滋蔓而抖然
自复者，是天地之邪与人之气血胶固充塞，郁勃纠纷，故复至三四次尚有陨命者矣。慎毋以其复
也而忽之。
伤寒者，为寒所伤，其来也有因，故初感总以汗散为主。若瘟疫并作因寒而得，不可以治伤寒
之法治之。非惟麻、桂不用，即羌活、十神等汤亦非对症之药。所谓读伤寒书不足以治瘟疫者此
也。至于瘟疫变现杂症之多，几与伤寒等。吴又可《瘟疫论》中，仅有斑、黄汗、狂等数条，至于
《伤寒》中之诸汗、诸痛、诸血症，以及谵狂、渴烦、惕 、螈 、不语、摇头、大小便等症之方
论，瘟疫中可以裁取而用之者，正复不少也。然必斟酌尽善而后，可是总在人之学力见解，而非口
说之所能尽矣。所谓不读伤寒书，不足以治瘟疫者如此。
伤寒书率皆将阴阳二症参错并举，倏言阳症而用硝、黄，又倏言阴症而用桂、附，推作者之
意，虽相提并论，而其中分析，原自了然，若曰阳症若此，而阴症则如彼也。读者不善体会，随
将阴阳二症搅作一团，故有谓一人之病，有忽阴而忽阳者，有谓病在阳经为阳症，传入阴经为阴
症者，有谓阴阳错杂而难分者，种种支离，不可枚举。即不出乎此，亦视阴症为世所长有，与阳
症参半，故临症每将阴阳二字交战于心，而迄无定见。无怪乎用药差错，而误人性命也。欲除此
弊，莫若分读，先习传经之阳症，将直中阴经之阴症，暂行缓看。盖阳症明，而习阴症自易易耳。
何者？阳症头绪繁多，变现百出，至于阴症，并无传变，治法无多，易学易疗，当黜之杂症门中，
与暑、湿、霍乱、诸中等疾为一类，则自无阴阳误治之弊。
凡是治疗伤寒和瘟疫，医生最重视的是初次患病的情况。至于“抄复”（指疫病痊愈后再次发作），认为死亡的人很少，往往把它看得很轻而漫不经心。这是因为认为“抄复”的病，人体的经络、脏腑都是前一次瘟邪曾经传遍过的地方，所以这一次不过是走熟路而行，因此邪气容易排出。古人原本就有这种说法，但哪里知道这只是说通常的情况呢？唯独在瘟疫盛行的时候就不是这样了。因为此时疫气积聚浓厚，即使没有饮食劳损的原因，也有因再次感受疠气而复发的；更有前番残留的邪气稍微没有清除干净，再经过酝酿滋蔓而突然复发的。这是天地的邪气与人的气血胶着固结、充塞体内，郁结勃发、纠纷缠绕，所以复发到三四次还有丧命的。千万不要因为它复发就忽视它。
伤寒，是被寒邪所伤，它的来路是有原因的，所以初感时总以发汗解表为主。如果瘟疫同时发生，是因寒而得的，就不能用治疗伤寒的方法来治疗它。不仅麻黄汤、桂枝汤不能用，就是羌活汤、十神汤等也不是对症的药。这就是所谓“只读伤寒书不足以治疗瘟疫”的原因。至于瘟疫变现出的杂症之多，几乎与伤寒相等。吴又可《瘟疫论》中，仅有斑疹、黄汗、狂躁等几条，至于《伤寒论》中的各种汗证、各种痛证、各种血症，以及谵语狂躁、口渴烦躁、惊惕、抽搐、不语、摇头、大小便异常等症状的方论，在瘟疫中可以斟酌选取而使用的，正不少啊。但是必须斟酌得尽善尽美才行，这总是在于医生的学识水平和见解，而不是口头说说就能穷尽的了。这就是所谓“不读伤寒书，不足以治疗瘟疫”的道理。
伤寒类的书籍大都将阴阳两种病症参差错落地并列在一起，一会儿说阳症而用硝石、大黄，一会儿又说阴症而用桂枝、附子。推究作者的本意，虽然是将两者相提并论，但其中的分析，原本自己心里是清楚的，意思是：阳症如果是这样，那么阴症就是那样。读者如果不善于体会，就会把阴阳两种病症搅成一团。所以有人称一个人的病，有忽阴忽阳的；有人说病在阳经为阳症，传入阴经为阴症；有人说阴阳错杂难以分辨，种种支离破碎的说法，不可枚举。即使不出于此，也认为阴症是世间常有的，与阳症各占一半，所以在临症时常常将“阴阳”二字在心中交战，始终没有定见。难怪用药出错，而误人性命。想要消除这种弊端，不如分开阅读，先学习传经的阳症，将直中阴经的阴症暂时搁置不看。因为阳症明白了，学习阴症自然就容易了。为什么呢？阳症头绪繁多，表现百出；至于阴症，并没有传变，治法不多，易学易疗，应当将其归入杂症门类中，与暑病、湿病、霍乱、诸中等疾病归为一类，那么就自然没有阴阳误治的弊病了。
Whenever treating *Shanghan* (Cold Damage) and *Wenyi* (Plague Epidemic), physicians place the greatest emphasis on the initial onset of the disease. Regarding "Chaofu" (recurrence of the epidemic after apparent recovery), it is often considered that few die, leading practitioners to view it as a minor condition and treat it with negligence. The reasoning is that in cases of recurrence, the meridians and zang-fu organs have already been traversed by the pathogenic factors during the previous infection; thus, this time the pathogen merely travels a familiar route, making it easier to expel. Ancient texts indeed contain such arguments, but these only describe the general rule. However, during periods of rampant epidemic, this is not the case. At such times, the epidemic qi accumulates densely. Even without causes related to diet or overexertion, there are recurrences due to secondary contraction of pestilential qi. Furthermore, residual pathogens from the previous episode may linger, undergo further fermentation and spread, and suddenly recur. This occurs because the external pathogenic factors adhere firmly to the patient's qi and blood, causing blockage, stagnation, and chaotic entanglement. Consequently, patients may suffer fatal outcomes even after three or four recurrences. One must never disregard the condition simply because it is a recurrence.
*Shanghan* is caused by cold injury, which has a specific etiology; therefore, the primary treatment for the initial contraction is always diaphoresis (sweating to release the exterior). If *Wenyi* occurs concurrently and is triggered by cold, it cannot be treated using methods for *Shanghan*. Not only should *Mahuang Tang* (Ephedra Decoction) and *Guizhi Tang* (Cinnamon Twig Decoction) be avoided, but also *Qianghuo Tang* (Notopterygium Decoction) and *Shishen Tang* (Ten Deities Decoction) are not appropriate remedies. This illustrates why "reading only *Shanghan* books is insufficient to treat *Wenyi*." Moreover, the variety of complex symptoms presented by *Wenyi* is nearly equal to that of *Shanghan*. In Wu Keyi’s *Wenyi Lun* (Treatise on Pestilential Diseases), there are only a few entries regarding macules, yellow sweat, and mania. However, the formulas and discussions in *Shanghan Lun* (Treatise on Cold Damage Disorders) concerning various sweating patterns, pains, bleeding disorders, delirium, mania, thirst, irritability, palpitations, convulsions, mutism, head shaking, and abnormalities of urination and defecation can be extensively adapted and utilized in the treatment of *Wenyi*. Nevertheless, careful deliberation is required to ensure perfection; this depends entirely on the physician's scholarly attainment and insight, which cannot be fully conveyed through mere words. This explains why "not reading *Shanghan* books makes one insufficient to treat *Wenyi*."
Books on *Shanghan* generally intermingle yin and yang syndromes. They abruptly mention yang syndromes treated with *Xiaoshi* (Mirabilite) and *Dahuang* (Rhubarb), then abruptly mention yin syndromes treated with *Guizhi* (Cinnamon Twig) and *Fuzi* (Aconite Root). Examining the author's intent, although they are discussed together, the internal analysis remains clear: if a yang syndrome presents thus, a yin syndrome presents otherwise. Readers who fail to grasp this nuance tend to confuse yin and yang syndromes into a single entity. Thus, some claim that a single patient's illness fluctuates between yin and yang; others assert that disease in yang meridians constitutes a yang syndrome, while transmission to yin meridians constitutes a yin syndrome; still others argue that yin and yang are mixed and indistinguishable. These fragmented views are numerous and varied. Even excluding these, many consider yin syndromes common in clinical practice, accounting for half of all cases. Consequently, when diagnosing, physicians often struggle internally with distinguishing yin from yang, lacking definitive judgment. No wonder medication errors occur, leading to fatal mistakes. To eliminate this flaw, it is best to study them separately: first master the transmitted yang syndromes, and temporarily postpone studying the direct-entry yin syndromes. Once yang syndromes are understood, learning yin syndromes becomes easy. Why? Yang syndromes are complex with diverse manifestations, whereas yin syndromes involve no transmission, have fewer treatment methods, and are easier to learn and treat. They should be categorized under "Complex Syndromes," grouped with summerheat, dampness, cholera, and other similar diseases, thereby avoiding the error of misdiagnosing yin and yang.