Da Shanghan (Major Cold Damage) refers to the condition where, during the severe cold of winter, water freezes and the ground cracks, forming a lethal qi (vital energy/pathogenic factor). Individuals with weak constitutions who are exposed to this pathogen will fall ill immediately upon infection, which is termed Zheng Shanghan (True Cold Damage). If illness does not occur immediately, the cold pathogen hides between the skin and muscles, lying dormant within Ying (Nutritive Qi) and Wei (Defensive Qi). When spring's warm air arrives, it may manifest as Wenbing (Warm Disease); if it manifests in summer due to summer-heat, it is called Rebing (Heat Disease). Although collectively referred to as Shanghan, they are essentially Heat Diseases. In treating Shanghan, one must first differentiate the Six Channels (Taiyang, Yangming, Shaoyang, Taiyin, Shaoyin, Jueyin), and only then proceed with diagnosis.

Upon pulse diagnosis, if the Cun, Guan, and Chi positions at both wrists are floating, and the pulse is floating, tight, and forceful, it indicates Shanghan; if it is floating, slow, and weak, it indicates Shangfeng (Wind Strike). Thus, the syndromes of the Taiyang Channel can be identified. After the Taiyang disease resolves, the pathogen transmits sequentially, inevitably entering the Stomach Channel (Yangming Channel). This typically occurs within one to three days, presenting with eye pain, nasal dryness, insomnia, tidal fever in the late afternoon (Ribu), aversion to heat rather than cold. This indicates pathology in the Taiyang and Yangming Stomach Channels. Pulse diagnosis reveals long pulses at all positions; a long and slightly surging pulse indicates Zang (Organ) disease, while a long and deep-rapid pulse indicates Fu (Hollow Organ) disease. Thus, Yangming syndromes can be identified.

After Yangming disease resolves, transmission proceeds to the Gallbladder Channel (Shaoyang Channel). Occurring within three to four days, symptoms include chest and hypochondriac pain, deafness, alternating chills and fever, vomiting, and bitter taste in the mouth. This indicates pathology in the Foot-Shaoyang Gallbladder Channel. Pulse diagnosis shows wiry pulses at all positions; a wiry, slippery, and rapid pulse indicates the onset of Jue (reversal/cold limbs), while a wiry and harmonious pulse indicates the disease is resolving. Thus, Shaoyang syndromes can be identified.

After Shaoyang disease resolves, transmission proceeds to the Spleen Channel (Taiyin Channel). Occurring around four to five days, symptoms include abdominal fullness or pain, dry throat, constipation, normal urination, or spontaneous diarrhea, warm hands and feet, and thirst. This indicates transmitted Fu-heat. If there is no headache or fever initially, but only aversion to cold, spontaneous diarrhea without thirst, and cold hands and feet, it is "Direct Invasion of Yin Channels," indicating pathology in the Taiyin Spleen Channel. Pulse diagnosis shows deep pulses at all positions; a deep, solid, and forceful pulse indicates interior excess-heat, while a deep, slow, and weak pulse indicates interior deficiency-cold. Thus, Taiyin syndromes can be identified.

After Taiyin disease resolves, transmission proceeds to the Kidney Channel (Shaoyin Channel). Occurring around five to six days, symptoms include dry mouth and tongue with thirst, and hands and feet that alternate between warm and cold. Constipation with delirium indicates heat; diarrhea with undigested food, no thirst, and blueish fingernails indicate cold. This indicates pathology in the Foot-Shaoyin Kidney Channel. Pulse diagnosis shows deep pulses at all positions; deep, solid, and forceful indicates heat, while deep, slow, and weak indicates cold. Thus, Shaoyin syndromes can be identified.

After Shaoyin disease resolves, transmission proceeds to the Liver Channel (Jueyin Channel). Occurring around six to seven days, symptoms include irritability and retraction of the testicles. Delirium with thirst indicates heat; salivation, lower abdominal pain without thirst, and body cold as ice indicate cold. This indicates pathology in the Foot-Jueyin Liver Channel. Pulse diagnosis shows deep pulses at all positions; deep, solid, and forceful indicates heat, deep, slow, and weak indicates cold, and floating-slow indicates spontaneous recovery. Thus, Jueyin syndromes can be identified.

What is Lianggan Shanghan (Double-Infection Cold Damage)? For example, on the first day, both Taiyang and Shaoyin channels are affected: headache indicates excessive Taiyang pathogen on the exterior; dry mouth and irritability/fullness indicate excessive Shaoyin pathogen on the interior. On the second day, both Yangming and Taiyin are affected: body heat and delirium indicate excessive Yangming pathogen on the exterior; anorexia and abdominal fullness indicate excessive Taiyin pathogen on the interior. On the third day, both Shaoyang and Jueyin are affected: deafness indicates excessive Shaoyang pathogen on the exterior; testicular retraction and cold limbs indicate excessive Jueyin pathogen on the interior. If the patient cannot take fluids and loses consciousness, death occurs after six days because one Zang and one Fu organ are simultaneously infected by the pathogen; this is why Lianggan is incurable. However, severity varies. If exterior syndrome is heavier than interior, treat the exterior first, then the interior. If interior is heavier, treat the interior first, then the exterior. This is a desperate measure to save life from death.

For those not suffering from double-infection and without transmission, by the seventh or eighth day, the pathogen in the primary channel is gradually suppressed, and by the twelfth day, mental clarity returns, and the disease heals spontaneously. However, transmission patterns vary and should not be rigidly fixed. Some start in Taiyang and transmit to Yin channels without involving Shaoyin; some skip Yangming and Shaoyang to involve Shaoyin directly; some bypass Yang channels entirely to enter Yin channels; some remain confined to one channel; some involve simultaneous infection of two or three Yang channels (Hebing/Combined Disease); some involve sequential infection of Taiyang and Yangming (Bingbing/Concurrent Disease). These cases require careful differentiation and treatment.

There is also Zhonghan (Cold Strike), caused suddenly by heaven-earth cold qi or ingestion of cold substances directly invading Yin channels. The six pulses are deep, weak, and powerless; the body is fatigued, hands and feet are cold, jaw is locked, and speech is impaired. This differs from Shanghan, where the pathogen penetrates gradually along the channels.

How to distinguish True Shanghan from Ganmao Shanghan (Common Cold-type Cold Damage) and Lei Shanghan (Shanghan-like conditions)? Han (Cold) is the severe, killing qi of winter. From Shuangjiang (Frost Descent) to Chunfen (Spring Equinox), any immediate illness after exposure is True Shanghan. If cold is contracted in winter but manifests after Chunfen, it is Wenbing (Warm Disease), with symptoms of fever, cough, body ache, thirst, no aversion to cold, wiry-rapid non-tight pulse, and right-hand pulse stronger than left due to internal constraint. If contracted in winter but manifests after Xiazhi (Summer Solstice), it is Rebing (Heat Disease), with surging-rapid pulse and more severe heat/thirst than in spring. Both Warm and Heat diseases feature thirst without aversion to cold, indicating internal heat reaching the surface. Aversion to cold suggests exposure to sudden abnormal cold, less severe than winter cold.

If illness arises from spring wind or summer summer-heat independently, it is distinct from Warm/Heat diseases. If contracted from epidemic qi with similar symptoms, it is Wenyi (Warm Epidemic), characterized by headache, body ache, chills/fever, and family contagion. Sudden redness/swelling of legs/knees or weakness/fever/headache is Hongxiangjiao Shanghan (Hongxiangjiao Cold Damage, i.e., Beriberi). Fullness in heart/abdomen, vomiting, headache, fever, aversion to cold, tight-full Guan pulse on right,平和 Pinghe (harmonious) pulses on left, and no body pain indicate Shangshi (Food Injury). Headache with body pain and tight pulses on both hands is Jiashi Shanghan (Shanghan with Food Retention). Tight left pulse, surging-slippery right pulse, or deep-fu Cun pulse, with fever, aversion to cold, vague headache, wheezing/cough, irritability, and chest/hypochondriac/body pain indicates Jiatan Shanghan (Shanghan with Phlegm). Headache, fever, aversion to cold, empty right pulse, and tight left pulse indicates Laoli Shanghan (Shanghan from Overwork). Tight-severe left pulse, deep-kou right pulse, pain in chest/abdomen, and sweat returning below knees indicates Xuxue Shanghan (Shanghan with Blood Stasis). Taiyang syndrome presents like mania; Yangming syndrome presents with forgetfulness; Shaoyang syndrome presents with chills/fever resembling malaria. These are Lei Shanghan (Shanghan-like), not true Shanghan.

Other miscellaneous disorders within Shanghan are numerous and listed in other sections. When miscellaneous disorders coexist with Shanghan, they are termed Shanghan Zazheng (Miscellaneous Disorders with Shanghan). Physicians should deduce the nature through clinical reasoning.