易病者，乃阴阳交易之谓也。盖大病方瘥，余邪未净，车取动淫欲，毒气返遏，互相为病也。若男病新瘥，
妇人与之交而反得病者，谓之阳易。妇病新瘥，男子与之交而反得病者，谓之阴易。若不因易而自
病重举者，名曰女劳复，通用逍遥散加减。在男子则阴肿，少腹绞痛。在妇人则里急腰胯重，连腹内
掣痛。其症热上□胸，气乏身重，头重不举，足不能移，眼中生花，四肢拘急，百节解散，男子卵缩入腹，
妇人痛引阴中，俱用烧？散，或豭鼠粪、竹皮等散，待小便利、阴头肿退为愈。若手足拳挛，舌吐，而脉
离经者，又为不治之症也。
伤寒汗不下彻，邪结在耳后一寸二分，或两耳下俱肿硬者，名曰发颐。此皆余邪热毒不清，速宜消
散，缓则肿溃矣，宜连翘败毒散。
夫阳气衰于下，则为寒厥。阴气衰于下，则为热厥。经云、阳气起于足五指之表，阳脉集于足下，而聚
于足心，故阳气胜则足下热。阴气起于足五指之里，阴脉集于膝下，而聚于足心，故阴气胜则从足
五指至膝上寒。其他伤寒时疫，热极入深、手足厥冷者，所谓热深厥亦深，阳极似阴也。若误认为寒，
而用热药，则杀人于俄顷矣。宜承气汤、白虎汤、大柴胡汤。若真是阴寒而厥者，则宜附子理中阳。阴
衰热厥，宜滋阴以制火。又有火郁脾土之中，手足热甚者，东垣升阳散火汤所由设也。
易病，是指阴阳之气相互交感传染而言。通常是大病刚刚痊愈，体内残余的病邪尚未清除干净，此时若进行房事，导致毒气返逆郁遏，从而互相致病。如果男子大病初愈，与女子发生性关系后反而患病，称为“阳易”。如果女子大病初愈，男子与她发生性关系后反而患病，称为“阴易”。如果不是因为交接而病情自行复发加重的，称为“女劳复”，通用逍遥散加减治疗。在男子身上表现为阴囊肿胀，少腹绞痛。在女子身上表现为腹部拘急、腰胯沉重，牵连腹内牵引作痛。其症状表现为热气上冲心胸，气息虚弱身体沉重，头部沉重难以抬起，双脚无法移动，眼前出现花影，四肢拘挛紧急，全身关节松懈无力，男子睾丸收缩入腹，女子疼痛牵引至阴部，均使用烧裈散（将贴身内裤烧成的灰），或豭鼠粪散、竹皮散等方剂，待小便通畅、阴头肿胀消退即为痊愈。如果出现手足蜷缩拘挛，舌头伸出不能收回，且脉象离经（异常紊乱）的，又属于不治之症。
伤寒病发汗不彻底，邪气结聚在耳后一寸二分处，或者双耳下方都出现肿胀坚硬的，称为“发颐”。这都是因为残余的热毒未能清除干净所致，应当迅速消散，拖延则会导致肿胀溃烂，适宜使用连翘败毒散。
阳气在下部衰微，就会形成寒厥。阴气在下部衰微，就会形成热厥。医经记载：阳气起始于足趾外侧，阳脉汇聚于足底并聚集于足心，所以阳气偏胜则足底发热。阴气起始于足趾内侧，阴脉汇聚于膝下并聚集于足心，所以阴气偏胜则从足五指到膝盖以上感到寒冷。其他如伤寒时疫，热邪极盛深入体内，导致手足冰冷的，就是所谓的“热深厥亦深”，即阳极似阴的表现。如果误认为是寒证而使用热性药物，则会立刻致人死亡。适宜使用承气汤、白虎汤、大柴胡汤。如果是真正的阴寒导致的厥证，则适宜使用附子理中丸（注：原文为“附子理中阳”，据中医常规及语境推测应为附子理中汤/丸类温阳方剂，此处保留原意译为温阳方剂）。阴气衰微导致的热厥，适宜滋阴以制约虚火。又有因火邪郁结在脾土之中，导致手足极度发热的，这就是李东垣设立升阳散火汤的原因。
Yi Bing (Disease of Exchange) refers to the transmission of pathogenic factors through the interaction of Yin and Yang. It typically occurs when a patient has just recovered from a severe illness, but residual pathogens remain in the body. If sexual intercourse is engaged in under these conditions, toxic qi returns and becomes obstructed, leading to mutual infection. If a male patient, newly recovered, contracts the disease after intercourse with a female, it is called "Yang Yi" (Yang Exchange). If a female patient, newly recovered, contracts the disease after intercourse with a male, it is called "Yin Yi" (Yin Exchange). If the disease recurs or worsens without such contact, it is termed "Nü Lao Fu" (Female Exhaustion Recurrence), which is generally treated with modified Xiaoyao San (Free and Easy Wanderer Powder). In males, symptoms include swelling of the genitals and colicky pain in the lower abdomen. In females, symptoms include abdominal urgency, heaviness in the waist and hips, and pulling pain extending into the abdomen. Clinical manifestations involve heat rushing up to the chest and heart, shortness of breath, heavy body, inability to lift the head, immobility of the feet, visual disturbances ("seeing flowers"), spasticity of the four limbs, laxity of all joints, retraction of the testes into the abdomen in males, and pain radiating to the vagina in females. These are treated with Shao Kun San (Burnt Underwear Powder), or formulas containing Jia Shu Fen (Ass's Dung) or Zhu Pi San (Bamboo Rind Powder). Recovery is indicated by the free passage of urine and the subsidence of swelling at the glans penis. However, if there is curling of the hands and feet, protrusion of the tongue, and a pulse that deviates from the normal course (Li Jing Mai), it is considered an incurable condition.
Fa Yi (Swollen Cheek/Jaw) occurs when sweating in Shang Han (Cold Damage) is incomplete, causing pathogenic factors to knot in the area one cun and two fen behind the ear, or causing hardness and swelling below both ears. This results from residual heat and toxins not being cleared. Prompt dispersal is required; otherwise, swelling will lead to suppuration. Lian Qiao Bai Du San (Forsythia Decoction for Resolving Toxins) is appropriate.
When Yang Qi declines in the lower part of the body, it results in Cold Jue (Reversal Cold). When Yin Qi declines in the lower part, it results in Heat Jue (Reversal Heat). The Classics state: Yang Qi originates from the lateral aspect of the five toes, and Yang meridians converge at the sole and gather at the center of the foot; thus, excess Yang leads to heat in the soles. Yin Qi originates from the medial aspect of the five toes, and Yin meridians converge below the knees and gather at the center of the foot; thus, excess Yin leads to coldness from the five toes up to the knees. In other cases, such as Shang Han or epidemic diseases, where extreme heat penetrates deeply causing cold hands and feet, this is known as "the deeper the heat, the deeper the Jue," representing "extreme Yang resembling Yin." Misdiagnosing this as cold and administering hot medicines can cause death instantly. Cheng Qi Tang (Pulse-Regulating Decoction), Bai Hu Tang (White Tiger Decoction), and Da Chai Hu Tang (Major Bupleurum Decoction) are suitable. For true Yin-cold Jue, warming Yang formulas like Fuzi Lizhong Wan (Aconite and Ginger Pill for Regulating the Middle) are indicated. For Heat Jue due to Yin deficiency, nourishing Yin to control fire is appropriate. Additionally, when Fire is constrained within the Spleen Earth, causing extreme heat in the hands and feet, this is the rationale for Li Dongyuan's creation of Sheng Yang San Huo Tang (Raising Yang and Dispersing Fire Decoction).