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When the smallpox rash appears, it is red. After five or six days, it suddenly changes to clear fluid (Qingjiang), then to white fluid (Baijiang), then to turbid fluid (Hunjiang), then to yellow pus (Huangnong), and finally forms a scab. Did the ancients consider the changes of smallpox to be clear fluid, white fluid, turbid fluid, and yellow pus? Smallpox is essentially turbid qi within the blood in the blood vessels. When encountering epidemic pestilence that expels turbid qi, it enters through the mouth and nose into the respiratory tract (Qi Guan), reaches the blood vessels, and drives the turbid qi and blood from the blood vessels, along with the body fluids (Jin Ye) from the respiratory tract, out through the hair pores. Thus, the lesions are round in shape and red in color. After five or six days, the blood within the smallpox lesions returns to the blood vessels, leaving only turbid qi and body fluids inside the lesions. Since the body fluids are clear, they are named "clear fluid" (Qingjiang). The clear fluid is refined by the pestilential toxin, becoming thick and white in color, hence named "white fluid" (Baijiang). The white fluid is refined again, becoming thicker and turbid, hence named "turbid fluid" (Hunjiang). The turbid fluid is refined once more, becoming as thick as pus from sores, hence named "yellow pus" (Huangnong). The yellow pus is dried up and forms a scab. The failure of smallpox to mature into pus (Xing Jiang) is due to the blood not returning to the blood vessels; the reason the blood does not return to the blood vessels is that the pestilential toxin refines within the blood vessels, causing blood coagulation and blocking the pathway of blood flow. If the stasis in the blood vessels is unblocked, why worry about the failure of pus formation?

For those who experience choking when drinking water four or five days or seven or eight days after the onset of smallpox, the ancients discussed this as toxic fire obstructing the throat and listed it among incurable conditions. This stems from a lack of understanding of the anatomy of the throat, specifically the left Qi Gate (Zuo Qi Men) and right Qi Gate (You Qi Men). At the root of the tongue, there is a white flap, dense like a coin, called the epiglottis (Hui Yan), which covers the opening of the trachea (Fei Guan) and the left and right Qi Gates. When swallowing food and drink, one must press the tip of the tongue against the hard palate, causing the epiglottis to tightly cover the trachea and the openings of the left and right Qi Gates, allowing food and drink to pass through the trachea and Qi Gates into the esophagus (Wei Guan) behind. Observe people eating: when food and drink enter the throat but have not yet reached the pharynx, if one suddenly laughs coldly, causing a violent upward rush of qi, the epiglottis opens. If a grain of rice or a drop of water enters the left or right Qi Gate, it will immediately choke out through the nose; this is the manifestation of the condition. Currently, pestilential toxin refinement causes blood coagulation in the epiglottis, preventing it from tightly covering the Qi Gates, thus causing choking when water seeps in. Eating does not cause choking because the tiny gap allows water to seep in but prevents food from entering, hence no choking. Resolving the blood stasis in the epiglottis will immediately stop the choking.