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The manifestations of epidemic pathogens are myriad and ever-changing, yet they ultimately do not go beyond the categories of Exterior (Biao) and Interior (Li). However, when Exterior patterns contain Interior pathogens, or Interior patterns contain Exterior pathogens, careful differentiation is indispensable. Therefore, listing patterns by distinguishing between Exterior and Interior covers their regularities, while detailed differentiation exhausts their variations, enabling practitioners to maintain clear judgment during clinical practice and thereby save lives from premature death in rare instances.

The fever caused by seasonal epidemics resembles that of wind-cold miscellaneous diseases; however, during fever, differences exist in Qi (vital energy), complexion, spirit, pulse, and tongue coating. Once identified as epidemic fever, one must further recognize distinctions in depth (shallow/deep) and location (Exterior/Interior); without such differentiation, treatment cannot be administered. Most fevers belong to the Exterior pattern, but there are also fevers belonging to the Interior pattern, half-Exterior half-Interior pattern, fevers arising from residual pathogens re-emerging at the Exterior after partial retreat, and fevers resulting from deficiency of healthy Qi after pathogenic factors have receded.

In cases of Exterior pattern fever, the pulse is neither floating nor deep but rapid; the Cun (inch) position is stronger than the Guan (bar) and Chi (cubit) positions; the heat sensation is superficial on the skin, feeling scalding to the touch initially but diminishing upon prolonged pressure; it is invariably accompanied by headache, stiffness of the neck and back, lower back pain, soreness of the shins, or redness, swelling, and pain in the head, face, body, or skin. These symptoms need not all appear simultaneously; the presence of any one indicates Exterior pattern fever. Formulas such as Jiuwei Qianghuo Decoction (Nine-Ingredient Notopterygium Decoction), Renshen Baidu San (Ginseng Detoxification Powder), or Liushen Tongjie San may be selected. In severe winter cold or cases with significant aversion to cold, Da Qinglong Tang (Major Blue-Green Dragon Decoction), Weirui Tang (Asparagus Decoction), Yebibo Tang (Yi Ma Decoction), or Yangdan Tang may be borrowed. In cases with no aversion to cold at all, Baihu Tang (White Tiger Decoction) or Huangqin Tang (Scutellaria Decoction) may be used with modifications.

In cases of Interior pattern fever, the pulse may be slippery, deep-rapid, or surging-slippery; the Guan and Chi positions are stronger than the Cun position; the heat sensation is located deep within muscles and bones, feeling mild upon initial touch but intense upon prolonged pressure; it is invariably accompanied by irritability and thirst, chest and abdominal fullness, constipation or spontaneous diarrhea, bloody or purulent stools, yellow-red urine, or delirium and manic stupor. Although these symptoms need not all appear, the presence of two or three confirms Interior pattern fever. Formulas such as Zhizi Chihui Tang (Gardenia and Soybean Fermented Decoction), Huanglian Jiedu Tang (Coptis Detoxification Decoction), Xiao Xiantong Tang (Minor Trapped Phlegm Decoction), Three Chengqi Decoctions (Three Bowel-Unblocking Decoctions), Daochi San (Guide Out Redness Powder), Xiexin Tang (Drain Epigastrium Decoction), Zhuling Tang (Polyporus Decoction), or Tianshui San may be selected.

In half-Exterior half-Interior fever, the pulse is often wiry; there is chest and hypochondriac fullness; fever may come and go; symptoms may include bitter taste in the mouth, dry throat, vertigo, deafness, red eyes, nausea, or irritability; or it may present with mixed Exterior and Interior signs. Darao Yin (Reach the Origin Infusion), Chaige Jieji Tang (Bupleurum and Kudzu Muscle-Releasing Decoction), or Xiaochaihu Tang (Minor Bupleurum Decoction) may be selected.

When treating fever caused by seasonal epidemics, clarity in medication strategy is paramount. If the initial approach is correct, subsequent transmission will be less critical and rescue easier; otherwise, the situation becomes difficult to manage. Upon observing fever, one must differentiate based on Qi, complexion, spirit, pulse, and tongue coating to determine if it is wind-cold or seasonal epidemic. If it is a seasonal epidemic, one must distinguish whether it is in the Exterior, Interior, or half-Exterior half-Interior. However, pure Exterior or pure Interior patterns are rare in epidemics; mixed Exterior-Interior patterns are common. For mixed patterns, Wu’s Darao Yin (Reach the Origin Infusion) is the primary formula. If Exterior signs predominate, add Qianghuo (Notopterygii Rhizoma et Radix); if Interior signs predominate, add Dahuang (Rhei Radix et Rhizoma); if half-Exterior half-Interior signs predominate, add Chaihu (Bupleuri Radix), Gegen (Puerariae Lobatae Radix), and Danshi (Sojae Semen Praeparatum); if both Exterior and Interior signs are present, use all herbs together, i.e., Sanxiao Yin (Three Eliminations Infusion), which yields the best results and serves as the principal formula for seasonal epidemics.

If fever recurs several days after recovery, it indicates residual pathogens lurking in the Mu Yuan (Membrane Source) re-emerging at the Exterior. One should assess the proportion of Exterior versus Interior signs and apply previous methods. Generally, post-recovery recurrence implies more Interior heat than Exterior heat; even if Exterior-releasing herbs are indicated, only Gegen (Puerariae Lobatae Radix), Chaihu (Bupleuri Radix), and Danshi (Sojae Semen Praeparatum) are appropriate, not Qianghuo (Notopterygii Rhizoma et Radix). This does not apply if the patient contracts new wind-cold after recovery, presenting with fever, anhidrosis, and a tongue lacking coating. For post-epidemic fever with anhidrosis, using a heavy dose (5 qian) of Gegen (Puerariae Lobatae Radix) is optimal because its cool nature releases the muscle layer and induces sweating without obstructing the Exterior (due to anhidrosis) or aggravating the Interior (due to irritative heat).

In patients with chronic deficiency, the elderly, or those contracting seasonal epidemics after severe illness, where repeated sweating, purging, and clearing treatments lead to intensified fever, or absence of solid Exterior/Interior signs, or hollow pulses (Huohuo Kong), or pulses more surging/slippery than at onset, or worsening body pain with Exterior-releasing herbs, or increased irritability/stupor with cooling herbs, or increased dry tongue with purgatives, these are fevers due to Healthy Qi deficiency after pathogen retreat. As Wang Taipu stated: "Great deficiency presents with excessive symptoms; wrongly applying purgation brings injustice." At this stage, one must disregard superficial symptoms and deliberate on Yin/Yang and Deficiency/Excess. Yin deficiency manifests as heat-thirst and exhaustion, rooted in the Kidney, treated with Liuwei Dihuang Wan (Six-Ingredient Rehmannia Pill); if combined with Qi deficiency, integrate Shengmai San (Generate Pulse Powder), prepared as strong decoctions taken frequently day and night for swift effect. Yang deficiency manifests as vomiting/diarrhea and palpitations/dizziness, rooted in the Spleen, treated with Liujunzi Tang (Six Gentlemen Decoction); if combined with Blood deficiency, select Guipi Tang (Spleen-Restoring Decoction), Canhu Sanbai San (Ginseng, Bupleurum, and Three Whites Powder), or Qingzao Tang (Clear Dryness Decoction). If such conditions are still treated with sweating, purging, cooling, or dispersing methods, survival is impossible.

Furthermore, when fever clearly presents as Exterior, Interior, half-Exterior half-Interior, recurrent, or deficient, treatment is easy. If pulse and symptoms are mixed and obscure, tongue coating becomes the key diagnostic basis. Initial thin white coating or moist coating without苔 indicates Exterior. Thick white coating, or slightly yellow, or yellow center with white edges/tip, or mixed colors, indicates half-Exterior half-Interior. Yellow, sauce-colored, or black coatings indicate Interior. A dry coating, regardless of color, indicates Interior. After repeated sweating/purging, a moist coating with fever indicates Yang deficiency; a coating-less dry tongue indicates Yin deficiency. Differentiating Exterior/Interior/Deficiency/Excess via these signs resolves most of the diagnostic challenge. However, Deficient fever mimicking Excess makes tongue diagnosis unreliable; one must trace the disease's progression. If fever worsens after repeated sweating/purging/dispersing, treat as Deficiency. If fever gradually decreases after sweating/purging, indicating efficacy, it remains residual pathogen; do not tonify hastily to avoid trapping heat and shortening life. Such critical junctures between Deficiency and Excess require careful discernment.

Although the above methods for differentiating Exterior/Interior/Deficiency/Excess focus on fever, they can be extrapolated to all patterns, depending on the practitioner's aptitude.