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The so-called Shanghan (Cold Damage) refers to the occurrence of a lethal pathogenic factor during winter when the weather is severely cold, water freezes, and the ground hardens. At this time, the Kidney and Bladder (belonging to Kan Water) are in their dominant phase. Individuals with deficient constitutions who contract this cold qi may fall ill immediately, which is termed Shanghan; if they do not fall ill immediately, the cold toxin hides within the skin and muscles. This occurs because the Kidney Water is depleted, preventing the Spring Wood Qi from generating, causing heat to fail in dissipating and stagnating internally. Upon encountering a trigger, it manifests: by Spring it transforms into Wenbing (Warm Disease), and by Summer it transforms into Rebing (Heat Disease), with Rebing being more severe than Wenbing. Although named Shanghan, it is essentially a Heat Disease.

If the climate does not follow the proper seasonal order—spring should be warm but turns cold, summer should be hot but turns cool, autumn should be cool but turns hot, and winter should be cold but turns warm—this abnormal climate appearing out of season causes people of all ages throughout the year to suffer from similar illnesses. This refers to the irregular Qi of Shixing (Seasonal Epidemic), rather than a violent lethal Qi. The violent lethal Qi refers to Yi Bing (Epidemic Disease). Yi Bing refers to sudden cold occurring between the Spring Equinox and before the Autumn Equinox, which constitutes a seasonal Cold Epidemic.

There are also the normal Qi of the four seasons: Spring Qi is mild and harmonious, Summer Qi is hot and sultry, Autumn Qi is clear and cool, and Winter Qi is biting and cold; these constitute the Normal Qi of the four seasons. However, Normal Qi can also cause disease: injury by Wind in Spring leads to undigested diarrhea (Sunxie) in Summer; injury by Summerheat in Summer leads to Malaria and Dysentery (Nuli) in Autumn; injury by Dampness in Autumn leads to Cough in Winter; injury by Cold in Winter leads to Warm Disease in Spring. All are collectively termed Shanghan.

Diseases enter from the exterior, either entering the Yang meridians or the Yin meridians. They do not necessarily start at the Taiyang (Greater Yang) and end at the Jueyin (Terminal Yin). Some injuries remain only in Taiyang; some transmit to one or two meridians and stop; some remain in a single meridian throughout; some transmit across meridians; some enter Taiyang without forming stagnant heat, then directly enter Shaoyin (Lesser Yin) becoming a Yin syndrome; some directly strike the Yin meridians becoming a Cold syndrome. Due to the lack of explicit text in classics, many physicians treat them erroneously. If Yang meridians transmit to Yin meridians forming a Yin syndrome, although there is external cold extremities (Jue Leng), there is internal Heat pathogen. If there is no fever, limbs become cold immediately, and there is aversion to cold, this is a Yin syndrome caused by direct strike to the Yin meridians. Generally, starting in the Qi aspect of the Three Yang meridians and transmitting to the Blood aspect of the Three Yin meridians results in deeper heat leading to deeper cold extremities. Is this not the principle that "excess brings harm, and control restores balance"? It is Heat extreme manifesting as Cold transformation. Fever followed by cold extremities indicates a Yin syndrome transmitted via meridians, for which Sini San (Frigid Extremities Powder) or Dachengqi Tang (Major Order the Qi Decoction) should be used to purge according to severity. If cold extremities occur initially without fever, this is a Cold syndrome from direct strike to the Yin meridians, for which Sini Tang (Frigid Extremities Decoction) should be used to warm.