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Epidemic diseases, termed "Yi" (役), are akin to corvée labor. They often occur in the aftermath of droughts, floods, or wars, where intense heat and humidity evaporate, causing the qi of corpses to mix with excessive pathogenic qi, transforming into virulent toxins that spread diffusely between heaven and earth. Those infected may suffer on a large scale affecting a commandery or city, or on a small scale affecting a village or town, transmitting mutually among themselves. Although the cause of infection is the same, the resulting symptoms vary. For instance, Su Dongpo discussed cold-damp epidemics; Li Dongyuan discussed deficiency-type epidemics; Wu Youke discussed epidemics caused by the struggle between dampness and heat; and Yu Shiyu discussed summer-heat and dryness epidemics. Therefore, Liu Wenshu recorded acupuncture methods for five types of epidemics in the *Suwen Yipian* (Lost Chapters of the Yellow Emperor's Inner Canon), and Pang Anchang recorded treatments for five-colored diseases in the *Zongbing Lun*. However, one must not adhere rigidly to these texts. Yue Ren (Bian Que) stated in the *Nan Jing* (Classic of Difficult Issues), Chapter 58, that "Shanghan" (Cold Damage) encompasses five categories: wind strike, cold damage, damp-warmth, heat disease, etc., describing their syndromes and pulses in detail. The pathogen travels through various meridians, making it unclear which specific meridian is affected. Treatment should generally be selected based on the location of the pathogen within the meridians. The virulent qi of heaven and earth cannot be predicted by ordinary principles nor treated by routine methods. Those who initiate sweating therapy upon seeing warm diseases are erroneously applying Shanghan treatment methods to warm diseases. Conversely, when encountering epidemic diseases, physicians often fail to use purgative methods when indicated; this is applying warm disease treatment methods to epidemics, which is an even greater error.

Fever
In cold syndromes, fever accompanied by aversion to cold indicates the disease has originated in the Taiyang syndrome (diseases of the Taiyang channel). Fever absent but aversion to cold present indicates the disease has originated in the Shaoyin syndrome (diseases of the Shaoyin organ system). Fevers in warm epidemics manifest as tidal fever (fever that occurs at regular intervals or worsens at specific times), unlike the high fever seen in Shanghan. Initially, there may be occasional aversion to wind, but by the next day, thirst and aversion to heat appear. Epidemic syndromes present with severe aversion to cold combined with high fever, resembling Dan Nue (a type of malaria characterized by high fever).

Vomiting and Diarrhea
When the Wood of the Shaoyang Gallbladder is rampant with fire, vomiting results from upward rushing of qi, and diarrhea results from downward pressure of qi; this is termed vomiting and diarrhea due to Wind-Warmth. If middle-jiao deficiency leads to diarrhea first, followed by vomiting due to retained fluids, this is termed vomiting and diarrhea due to Damp-Warmth. Regarding vomiting and diarrhea caused by failing descent of Stomach Qi and disharmony of Lung Qi, these are common in epidemics. Generally, whether in Warm Diseases or Epidemics, the presence of vomiting and diarrhea represents an exit route for the pathogen and should not be abruptly stopped; this contrasts with cold diseases.

Various Pains
Headache and eye pain in Taiyang-Yangming Shanghan do not typically cause difficulty in turning the head or lifting it. In contrast, Warm Diseases and Epidemics cause headaches as if splitting, blurred vision, and a condition so severe it seems unbearable, accompanied by bone distress and pain, and waist pain as if beaten by a stick. Cold diseases are attributed to injury of Yang Qi, whereas Warm Epidemics are attributed to depletion of Yin Fluids.

Cold Limbs
In cold syndromes, cold limbs result from the obstruction of Kidney Yang. In Warm Syndromes and Epidemic Syndromes, they result from the failure of Liver Yang to circulate freely. Thus, cold pathogens occupying the Shaoyin differ from heat pathogens latent in the Jueyin.

Epistaxis
In cold syndromes, epistaxis indicates that the pathogen is about to retreat. In Warm Epidemics, epistaxis indicates that the pathogen is advancing.

Blood Accumulation
If a cold syndrome requires sweating but does not receive it, heat becomes bound in the Bladder. If an epidemic requires purgation but fails to receive it, fire becomes stagnant in the Membrane Source (Moyuan). Both scenarios can lead to Blood Accumulation. Any case presenting with persistent fever, free urination, mental restlessness resembling mania, and forgetfulness constitutes Blood Accumulation.