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The Kidney is a Water Zang organ. Water corresponds to the North, originating from "Heaven generates One (Water)." Thus, at the initial stage of receiving Qi, two Kidneys are formed: the left is the Kidney, belonging to Water; the right is the Mingmen (Gate of Life), belonging to Fire. This is referred to as "without Water, there is no growth; without Fire, there is no formation," where Water and Fire mutually support each other, representing the manifestation of Yin and Yang.

However, the Kidney is located inferiorly. At the onset of variola (smallpox/varioloid), if only the ears are cold, it is because the Bladder is the Fu organ of the Kidney, and the Kidney has not been invaded by pathogenic factors. This demonstrates the Kidney's ability to preserve its Water virtue to control Yang heat. If the ears become hot instead, it indicates that Water fails to control Fire, leading to the danger known as "returning to the Zang organs" (internal collapse of toxins). Treatment should aim to suppress rampant fire and nourish Kidney Water. Moreover, Yang often has excess while Yin is frequently deficient. Consequently, true Water may be exhausted, and body fluids suddenly depleted, causing Qi stagnation. The skin and hair lose moisture, and the interstices (couli) fail to receive nourishment. The blood within the pox becomes dry and turns black. Blackness is the color of dried blood resulting from fiery dryness and water depletion. Lower back pain is a sign of excessive toxic fire harming the body. This is a common principle. Although black belongs to Gui Water (Kidney Water), one cannot rigidly adhere to the notion of Kidney Excess syndrome and apply purgative methods.

The condition of blackening accompanied by lower back pain essentially stems from extreme heat and flourishing fire, as stated in the classics: "Excess leads to harm." External fire scorches the skin and flesh, causing black discoloration; the mutual excess of fire and toxin depletes the profound Water (Kidney Yin), resulting in lower back pain. The pox will inevitably be dry and withered, or present with numerous macules/papules, appearing either red or purple, or purple turning black. This clearly indicates a Fire pattern. The treatment method should primarily employ lifting and dispersing techniques to release the exterior, combined with cooling and detoxifying approaches. At the initial appearance of the spots, allowing the heat-toxin to resolve and the purple macules to clear, so that normal pox can emerge, followed by regulating Qi and Blood, may salvage the patient. However, if the macules do not recede, the purple hue does not clear, the pox remains hidden and fails to erupt, lower back pain persists making extension difficult, the head and face swell prematurely, and abdominal distension with coarse breathing occurs, it implies the pox will ultimately fail to rise, and survival is unlikely. Furthermore, if the pox, nearing maturity, suddenly turns black and undergoes inversion (inverted eruption/collapse), this is also caused by blood heat and hyperactive fire, where toxin stagnation dries the blood, constituting an adverse internal attack. Therefore, Zhuang stated: "Macules and papules with inverted eruption presenting in black are called 'Ghost Sores'," which is a term expressing aversion. Is the theory that severe winter cold causes toxins to return to the Kidney and turn black reasonable?