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When Wind-Cold pathogen is in the Exterior, the tongue usually has no coating; even if there is a white coating, it is thin and slippery. As the pathogen gradually transmits to the Interior, the coating changes from white to yellow, from yellow to dry, and from dry to black.
In Plague (Yi Wen), upon the appearance of headache and fever, a white coating appears on the tongue immediately, which is thick and not slippery; or the color is mixed with pale yellow; or it is coarse like accumulated powder. If the pathogen transmits through the meridians into the Stomach, the coating may present two or three colors simultaneously.
There are also cases where the white coating is dry, or turns black without becoming dry. Generally, when the epidemic pathogen enters the Stomach, the tongue coating resembles that of Wind-Cold, but due to the presence of Dampness, it does not become dry. The key distinction lies in the Exterior stage: the white coating is thick,
which differs from Shang Han (Cold Damage). By distinguishing this at the Exterior stage, one should not use Acrid-Warm Diaphoresis (Xin Wen Fa San) for the Exterior, but rather employ Cooling and Purging methods once the pathogen enters the Interior; this approach ensures success.

After the seasonal plague transmits to the Interior, blood stasis accumulates significantly, and treatment follows the principle of purging the Interior, which will not be discussed in detail here. However, for patients who already have internal injury with stagnant blood and subsequently contract the seasonal plague, during the first one or two days of onset, all exterior symptoms of the plague are present,
yet the pulse may be Khong (Floating, large, and hollow, like pressing a green onion stalk) or Se (Rough and choppy, like a light knife scraping bamboo), resembling a Yang syndrome presenting with a Yin pulse. One must carefully inquire about pain in the chest, abdomen, hypochondrium, and limbs that cannot be pressed and feels soft upon palpation; this is definitive evidence of blood stasis.
The Khong and Se pulses here do not represent a Yin pulse appearing in a Yang syndrome, but rather an Interior pulse appearing in an Exterior syndrome. The treatment method must include resolving blood stasis by adding herbs such as Hong Hua (Carthami Flos), Tao Ren (Persicae Semen), Gui Wei (Radix Angelicae Sinensis Caulis), Chi Shao (Radix Paeoniae Rubra), and Yuan Hu (Rhizoma Corydalis), selecting one or two ingredients in appropriate doses. Only then can the Exterior pathogen be easily resolved, and the Se and Khong pulses easily restored. If the Khong and Se pulses are mistakenly identified as Yin pulses and warm tonifying agents are administered, mild cases may worsen, and severe cases may become critical.

Seasonal plague is inherently more difficult to treat than Wind-Cold because Wind-Cold transmission follows a specific sequence, whereas seasonal plague transmission occurs without fixed meridian patterns. Wind-Cold pathogens in the Exterior dissipate immediately after diaphoresis, while plague pathogens disperse only to reaggregate, repeatedly multiple times.
Wind-Cold interior syndromes harmonize after a single purgative intervention, whereas plague interior syndromes tend to recur after purgation, sometimes requiring one or twenty purgations; this is why seasonal plague is difficult to treat.
Patients with Spleen Deficiency are even more difficult to manage. Seasonal plague requires both sweating and purging to resolve, but patients with Spleen Deficiency cannot sweat effectively from the Exterior and cannot tolerate purgatives in the Interior. Even if sweating occurs, Qi may collapse following the sweat;
even if purgation occurs, Qi may collapse from below. In treating such cases, forced sweating should be avoided; when releasing the Exterior, one must concurrently support Zheng Qi (Upright Qi), using Ren Shen Bai Du San (Ginseng and Imperata Decoction) as an example. Light purgation should be avoided; when purging the Interior, one must concurrently secure Qi and generate body fluids, using Huang Long Tang (Yellow Dragon Decoction) as an example. The external manifestations of these cases show little difference, except that the pulse is not resilient to pressure. However, pathogens advance and retreat; when the pathogen is advancing and vigorous, the pulse may still feel forceful upon pressure, so one must not adhere rigidly to this sign. It is essential to integrate Qi, complexion, spirit, demeanor, and pulse signs. For instance, sallow complexion, listlessness, weak and rapid breathing, palpitations, and tinnitus are all signs of Spleen Deficiency with failure of Middle Qi to rise; comprehensive assessment of the whole body is necessary. If the whole body presents excess signs but one or two deficiency signs appear, the deficiency signs become critical; if the whole body presents deficiency signs but one or two excess signs appear, the excess signs become critical. Ultimately, one must weigh Biao (Branch/Surface) and Ben (Root/Origin). Syndromes belonging to the Exterior, Upper Jiao, or Six Fu Organs are considered Biao; syndromes belonging to the Interior, Middle Jiao, Lower Jiao, or Five Zang Organs are considered Ben. If excess resides in the Biao and deficiency in the Ben, the deficiency is heavy; if deficiency resides in the Biao and excess in the Ben, the excess is heavy. At this juncture of deficiency and excess,
careful deliberation is required to avoid error.