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The cause of plague (Yi, epidemic disease) was formerly believed to be due to "Qi present when it should not be," meaning spring should be warm but is instead severely cold, summer should be hot but is instead severely cool, autumn should be cool but is instead severely hot, and winter should be cold but is instead severely warm. This is because changes in wind, rain, yin, and yang cause slight损益 (increase or decrease) in climate; for instance, autumn heat implies more sunny days, and spring cold implies more rainy days. Compared to this, these are normal phenomena of Heaven and Earth and do not necessarily lead to frequent epidemics.

Shanghan (Cold Damage) and Zhongshu (Heat Stroke) result from contracting the normal Qi of Heaven and Earth. Plague, however, results from contracting Liqi (pestilential Qi). The abundance or scarcity of this Qi varies by year, its density or thinness varies by region, and its flourishing or declining state varies by season. When this Qi arrives, regardless of age or strength, anyone who contacts it will fall ill. The pathogen enters via the mouth and nose. Its residence is neither inside the Zang-Fu organs nor outside in the meridians, but rather lodges within the Fuji (hidden spine), close to the surface but near the Stomach. This is the boundary between exterior and interior, known as Banbiao Banli (Half-Exterior Half-Interior), which corresponds to what the *Zhenjing* (Classic of Acupuncture) calls "Henglian Moyuan" (transversely connecting the Membrane Source).

The Stomach is the Sea of the Twelve Meridians, and all Twelve Meridians converge at the Stomach. Therefore, Stomach Qi can distribute throughout the Twelve Meridians, nourishing the entire body, penetrating every hair and fiber. Generally, pathogens in the meridians constitute an Exterior syndrome, while those in the Stomach constitute an Interior syndrome. Now, since the pathogen resides in the Moyuan (Membrane Source), it is exactly at the junction where the meridians and the Stomach intersect; thus, it is called Half-Exterior Half-Interior. If the pathogen floats to the Yangming Meridian, there will be eye pain, pain in the supraorbital ridge, and nasal dryness. If it floats to the Shaoyang Meridian, there will be hypochondriac pain, deafness, alternating chills and fever, vomiting, and a bitter taste in the mouth. Broadly speaking, invasion of the Taiyang Meridian is most common, followed by Yangming, and then Shaoyang.

Pathogens attach to humans through Tian Shou (airborne transmission) and Chuan Ran (direct contact transmission). Although the mode of infection differs, the pathology is the same. The Qi of human mouth and nose connects with the Qi of Heaven. If one's Ben Qi (root Qi) is abundant, pathogens find it difficult to enter. If Ben Qi is deficient, external pathogens take advantage during respiration. In the past, three people traveled early in the morning through fog: the empty-stomached person died, the person who drank alcohol became ill, and the well-fed person remained healthy. Is it not similar for how plague pathogens attach? However, if the annual Qi is extremely fierce, regardless of constitution, anyone whose Zheng Qi (Upright Qi) is slightly declined will fall ill upon contact, disregarding the aforementioned dietary conditions.

Deep infection leads to immediate onset. Shallow infection means the pathogen cannot overcome the Zheng Qi, so it does not manifest immediately. It may only erupt after factors like hunger, fullness, fatigue, worry, anger, etc., injure the Zheng Qi, allowing the pathogen to spread. The mechanism of Ying and Wei (Nutritive and Defensive Qi) circulation becomes blocked, causing the body's Yang Qi to become constrained, resulting in fever. Initially, Yang Qi is rejected internally and cannot reach the exterior, leading first to severe aversion to cold, and in severe cases, cold extremities (Jue Ni). As Yang Qi gradually accumulates, depression reaches its peak and then opens up, reversing the cold extremities and causing heat throughout the interior and exterior. At this stage, there is only fever without aversion to cold because Yang Qi has circulated fully. Sweat should occur here; if absent, it depends on the severity of pathogen binding. Even if sweat occurs, it is merely superficial. For exterior pathogens in the meridians, one sweating episode resolves the condition. But now that the pathogen is in the Half-Exterior Half-Interior, although there is sweat on the surface, it vainly depletes True Qi, and the deeply lodged pathogen cannot be resolved. One must wait for the latent pathogen to recede gradually, allowing Surface Qi to move inward, triggering a major battle. Jing Qi (Essence Qi) moves from the interior to the exterior via the Moyuan. After shaking and trembling cease, fever returns. At this moment, interior and exterior connect, leading to profuse sweating, soaking clothes and bedding, and the pathogen resolving through sweat. This is called Zhan Han (Battle Sweating). Immediately afterward, the pulse should calm, the body cool, and the spirit clear, resulting in sudden recovery. There are also cases of resolution through spontaneous sweating; as long as the exterior releases, it is a favorable progression, and self-healing occurs even without medicine.

If the latent pathogen has not receded, the sweat produced only allows Wei Qi to gradually circulate, temporarily reducing heat, which rebounds after time passes. Tidal fever in the afternoon indicates severe depression at that time, related to the waxing and waning of Yang Qi with the seasons. Subsequent intensification of heat without aversion to cold is due to Yang Qi accumulation. Mild or severe aversion to cold depends on the strength or weakness of the person's Yang Qi. Duration of fever, whether continuous day and night or slightly reduced at dawn, depends on the severity of the contracted pathogen.

Plague pathogens resemble Nuojing (Malaria) pathogens, but Malaria does not transmit to the Stomach, whereas Plague does. Both begin with severe aversion to cold followed by fever, unlike Shanghan where fever accompanies aversion to cold. Once the latent pathogen activates, transformation syndromes appear. Transformations may resolve from the exterior or sink into the interior. Resolution from the exterior is favorable; sinking into the interior is unfavorable. Furthermore, the sequence of exterior and interior involvement varies: some involve exterior first then interior, others interior first then exterior, some only exterior without interior, some only interior without exterior, some show predominance of either exterior or interior, some transmit simultaneously to both, some recur in the exterior, some recur in the interior, and some transmit to both and then further subdivide.

Resolution from the exterior manifests as macules (Ban), Battle Sweating, Manic Sweating, Spontaneous Sweating, or Night Sweats. Sinking into the interior manifests as chest and diaphragm oppression, epigastric distension, abdominal pain, dry constipation, heat-binding with purging of watery stool, diarrhea with heat, vomiting, nausea, delirious speech, yellow tongue coating, black tongue coating, or prickles on the tongue. By understanding the symptoms, one knows the transformation; by knowing the transformation, one determines the treatment. This is a general overview; detailed discussions are found in the sections on Pulse, Symptoms, and Treatment Methods.