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Nuo is an ancient ritual. The origin of epidemics is very old. Why is Zhang Zhongjing's *Treatise on Cold Damage* (*Shanghan Lun*) not complete? It only mentions warm diseases (*Wenbing*) and heat diseases (*Rebing*), with not a single word mentioning epidemics (*Yi*). Therefore, later generations lacked effective treatment methods. Only in recent times have figures such as Yu Jiayan, Wu Youke, and Zhang Jingyue provided thorough discussions and detailed treatments, uncovering insights previously unexplored by predecessors. However, Zhang Jingyue favored the sweating method (*Hanfa*), Wu Youke favored the purgative method (*Xiafa*), and Yu Jiayan favored using aromatic herbs to expel turbidity (*Fangxiang Zhu Hui*). All three are renowned masters, yet why are their treatment methods so vastly different? It must be understood that all three treatment methods are appropriate, but the types of epidemics they treated were different.

The epidemic discussed by Zhang Jingyue refers to pathogens of the Six Excesses (*Liuyin*: wind, cold, summer-heat, dampness, dryness, fire) and abnormal seasonal qi. Its manifestation is similar to cold damage (*Shanghan*), so he often discusses them together, primarily using the sweating method to fully exploit its efficacy. Zhang Jingyue’s writings are precise and comprehensive.

The epidemic discussed by Wu Youke involves excessive heat qi entering through the mouth and nose, lying latent at the *Moyuan* (the boundary between exterior and interior). Since the pathogen is neither in the exterior nor the interior, it cannot be reached by sweating; thus, there are warnings against forced or harsh sweating. The stomach is close to the *Moyuan*, so pathogens enter the interior rapidly, necessitating urgent and repeated purgation. To understand the transmission of epidemic pathogens, Wu Youke’s discussion is the most detailed. However, the epidemic described by Wu Youke is the common epidemic of the four seasons, commonly known as "seasonal qi syndrome" (*Shiqi Zheng*).

In contrast, the epidemic discussed by Yu Jiayan arises after wars and famines, where disease qi and corpse qi mix with the abnormal qi of heaven and earth, compounded by the intertwining steaming of warm, hot, summer-heat, and dampness during spring and summer. People in this atmospheric intersection have no refuge, leading to continuous transmission throughout communities, becoming a great epidemic. This foul qi enters through the mouth and nose, travels directly through the central pathway, and spreads through the Sanjiao (Triple Burner). It is neither in the exterior nor the interior; sweating fails to resolve it, and purgation leaves it lingering. Therefore, the primary treatment is using aromatic herbs to expel turbidity, combined with detoxification. Thus, the treatments of these three masters each fit their respective indications, and one should not rigidly adhere to one while criticizing another.

For this condition, we also reference these three methods for treatment, divided into two pathways. Sweating and purgation correspond to the exterior-interior treatments of cold damage. The method of expelling turbidity and detoxifying addresses epidemic pathogens latent in the Sanjiao, which are neither in the exterior nor the interior. Listed below, the major principles for treating epidemics are established.

Commentary by Shao: The epidemic symptoms discussed by Zhang Jingyue refer to cold damage occurring in three seasons due to abnormal qi, named "Three-Season Cold Epidemic." Pathogens enter via the skin and muscles, so treatment follows cold damage principles, initially using sweating. If referring to the epidemic symptoms discussed by Wu Youke, these involve damp-warmth, sha, summer-heat, and heat pathogens entering through the mouth and nose, lying latent at the *Moyuan*. The *Moyuan* is the membrane connecting the spleen and stomach, serving as the semi-exterior semi-interior boundary for these organs. Pathogens latent here enter the stomach rapidly. Since the pathogen is in the stomach, purgation is the treatment. This represents warm-heat and damp-turbid pathogens invading the Six Meridians and transmitting to the Yangming Stomach Fu organ.

This is the common epidemic of the four seasons, i.e., damp-warmth, miasma, and seasonal qi syndromes, which are distinctly different from toxic plagues and heavenly行的 epidemics. What Zhang Jingyue describes is immediate exposure to warmth with pathogens already entering the stomach; hence, he advocates sweating, while Wu Youke advocates purgation. These two can be treated via the Foot Meridians, consistent with cold damage theory, forming one pathway. Yu Jiayan’s discussion concerns true severe epidemics caused by abnormal heavenly qi, toxic miasmas, and mixed disease/corpse qi, intertwining and steaming. Humans inhale this qi through the mouth and nose, traveling the central pathway and spreading through the Sanjiao, often causing blockage. Thus, aromatic expulsion of turbidity and detoxification/opening methods are used. This toxic plague pathogen differs from the previous two conditions and does not follow cold damage treatment methods. Expelling turbidity and detoxification form a unified pathway, constituting the second major approach. Severe epidemics possess a unique toxic qi that is highly contagious, often affecting entire families and communities continuously. Although the three masters’ discussions differ, each has specific indications. One must examine symptoms according to the case and apply suitable treatments, avoiding rigid adherence to a single view.

When elders and children are infected similarly, and the masses are affected uniformly, this is the circulation of epidemic qi, commonly called "seasonal qi." It is foul qi from heaven and earth entering through the mouth and nose. Some cases involve concurrent Six Excesses, others do not. Some enter the Foot Meridians presenting as exterior-interior patterns, while some enter the Hand Meridians where sweating and purgation are contraindicated. Treatment must be differentiated. The key to differentiation lies in the tongue.

Commentary by Shao: The Six Excesses are pathogens of wind, cold, summer-heat, dampness, dryness, and fire, which are normal seasonal qis. During major epidemics, these may be accompanied by cold or summer-heat, referred to as "concurrent Six Excesses." In essence, if the disease is purely plague-related, one must distinguish whether the pathogen entered via the skin/muscles (entering Foot Meridians, allowing sweating/purgation) or via the mouth/nose (entering Hand Meridians, requiring Sanjiao-based treatment focusing on Heart Ying and Lung Wei, with sweating/purgation contraindicated).

If the tongue coating changes from white to yellow, and then to black, the epidemic pathogen is moving from exterior to interior; sweating and purgation are indicated.

Commentary by Shao: Exterior pathogens present with white tongue coating; interior pathogens present with yellow coating; deep heat in Fu organs presents with black coating. For pathogens in Foot Meridians, differentiate exterior/interior to apply sweating/purgation.

If the tongue coating changes from white to bright red, the epidemic pathogen is moving from Wei (Defensive) to Ying (Nutritive), entering Hand Meridians rather than Foot Meridians. Sweating and purgation are contraindicated; expelling turbidity, detoxifying, and clearing the Ying level are indicated (New Method).

Commentary by Shao: Pathogens in Lung/Wei present with white coating; entry into Heart Ying presents with red tongue; transmission to Pericardium presents with bright red tongue with prickles. For Hand Meridian pathogens, treat based on Heart/Lung/Wei principles, differentiating Upper/Middle/Lower Sanjiao, and avoid Foot Meridian sweating/purgation methods.