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In that year, the epidemic qi (pathogenic factors causing infectious diseases) prevailed, and the illnesses suffered were severe with high contagiousness; even children knew to call it "epidemic" (Yi). As for mild epidemics, they seemed non-existent, due to the varying density of the toxic qi accumulation.

In another year, the epidemic qi was weak, with only a few people in the neighborhoods affected, and no transmission occurred. Contemporary physicians named it "Shanghan" (Cold Damage), and those unaware naturally did not call it an epidemic, while those who knew also hesitated to label it as such. So how could one identify it as an epidemic? Because the pulse and symptoms were identical in every detail to those seen during years of prevalent epidemics, and the therapeutic efficacy of medications was indistinguishable. Thus, it is known that Warm Epidemics (Wenyi) occur throughout all four seasons and persist year-round, differing only in degree and severity.

In years when epidemic qi does not prevail, mild cases may still occur. The general public calls them "Ganmao" (Common Cold), unaware that they are actually epidemics. If dispersing agents (Di-san agents used to induce sweating and release the exterior) are used, although they do not strictly match the pathology, there is little harm; the epidemic resolves spontaneously. This is not due to the medicine itself, as the condition would heal without treatment even so. In slightly more severe cases, misusing dispersing methods causes relatively minor harm. However, if tonifying or cold-cooling agents are mistakenly used, chronic stubborn diseases may result; this distinction must not be overlooked.

The so-called "Zaqi" (Miscellaneous Qi), though described as originating from Heaven and Earth, is actually derived from local terrestrial qi. This qi rises from the ground; where there is this qi, there is this disease. Just as it is said that Heaven and Earth generate all things, yet these things depend on local origins. Plants rely on rain and dew to grow, while animals rely on food and drink to nourish themselves. First comes the qi, then the matter. Extending this logic, infinite qi gives rise to infinite matter. However, the essence of Yin and Yang and the Five Elements inevitably undergo generation, restriction, transformation, and control. Therefore, all things have their preferences and taboos: what is favorable benefits, and what is taboo harms; harm implies restriction. Thus, all things are mutually restricted, such as cats restricting mice, or mice restricting elephants. Knowing that matter can restrict matter, one should understand that qi can also restrict matter. Examples of qi restricting matter include crabs dying in fog and jujube trees withering in fog; here, tangible qi restricts both animal and plant matter. Regarding intangible qi, which predominantly affects animals, consider cattle plague, sheep plague, chicken plague, and duck plague—are humans the only ones affected by epidemics? Yet, cattle get sick while sheep do not, chickens get sick while ducks do not, and humans get sick while livestock do not. This difference in injury arises because the qi varies. Recognizing that the qi differs leads to the term "Miscellaneous Qi." Matter is the transformation of qi, and qi is the change of matter; qi is matter, and matter is qi. Understanding qi allows one to understand matter, and thus understanding that matter can restrict qi. Medicines are substances that can restrict qi. For instance, slugs (Yanyou) resolve the toxicity of centipedes, and cat meat treats ulcers caused by rat bites. These are illnesses caused by receiving matter-qi, treated by using the qi of matter to counteract the qi of matter—a phenomenon that can be deduced. However, for illnesses caused by receiving intangible miscellaneous qi, it is unknown what substance can restrict it. Precisely because the specific restricting agent is unknown, physicians reluctantly resort to the three methods of Sweating (Han), Vomiting (Tu), and Purging (Xia) to resolve the condition. Alas! Even these three methods are not perfect, let alone knowing the specific substances. If one could know how to use matter to restrict qi, a single disease would be cured by a single herb reaching the target, eliminating the need for complex formulations involving Sovereign, Minister, Assistant, and Courier herbs and their adjustments.