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The eruption of smallpox (or similar exanthematous diseases) is desired to be sparse; sparsity implies less pathogenic toxin, whereas density implies severe toxin. This is an established principle from ancient to modern times.

There are specific preferences and taboos regarding the distinction between sparse and dense eruptions. For instance, the head and face should be sparse, as the head is the meeting point of all Yang meridians and must not be invaded by pathogens; the neck and throat should be sparse, so that the vital passages remain unobstructed; the chest and back should be sparse, as these areas house the Back-Shu and Front-Mu points of the Zang-Fu organs. However, for the hands and feet, density is not taboo.

"Sparse" does not merely mean few in number; if the eruptions are well-distributed, uniform in size, and clean, they can also be described as sparse. "Dense" does not necessarily mean numerous; if the eruptions cluster together, adhere to one another, and overlap layer upon layer, even if present in only a few locations, they can be described as dense.

Sparse eruptions may still indicate a poor prognosis in several scenarios: First, when initially appearing, only two or three red spots are seen, and the fever subsides quickly, seemingly indicating sparsity. However, if high fever persists, lips and mouth become dry and cracked, urination and defecation are obstructed, and there is restlessness and irritability, or if there is no body heat but increased thirst and irritability, this indicates that the toxin is severe and stagnant within, unable to erupt rapidly. Over days, eruptions appear gradually and become denser. This is the first scenario. Second, initially, only one or two eruptions appear with blackened, necrotic tips, or three or four clump together, visible in only a few places with none elsewhere. The patient sleeps excessively, refuses food, feels irritable and thirsty, and has difficult urination and defecation. This indicates toxin latent within; if delirium occurs, death may ensue before the eruptions fully develop. If they do develop, carbuncles and toxic sores will likely follow later. This is the second scenario. Third, although the eruptions are genuinely sparse, negligence arises, leading to failure to observe dietary and lifestyle restrictions, exposure to wind-cold, dietary injury, or inappropriate use of decoctions and pills, resulting in unpredictable complications. This is the third scenario.

Conversely, dense eruptions may have a favorable prognosis if treated early, cared for strictly, with no internal pathology, allowing all lesions to mature properly.

One often observes that morning-glory grasses wither by evening, while pines and cypresses remain resilient through winter. Plants differ in toughness and brittleness; tough ones resist damage, while brittle ones are easily injured. How much more so for humans, whose constitutions vary in robustness and whose Qi varies in strength? The toxin of skin eruptions favors "mature/old" (Lao) states and dislikes "tender/young" (Nen) states. A dark, waxy complexion versus a tender, bright red one reflects the maturity of color. A tight, solid form versus a flaccid, floating form reflects the maturity of shape. Thick, turbid pus versus clear, pale pus reflects the maturity of fluid content. Hard, thick scabs versus soft, thin scabs reflect the maturity of the crust. The factor governing maturity is the Defensive Qi (Wei Qi). As stated in the classics: "Defensive Qi warms the muscles, fills the skin, plumps the interstices, and controls the opening and closing of pores." Therefore, when Defensive Qi is strong, muscles are firm, skin is thick, interstices are dense, and pore regulation is normal. This allows for containment and restriction, preventing the toxin from running rampant. Consequently, the color becomes dark and waxy, the form tight and solid, the pus thick and turbid, and the scabs hard and thick. Such eruptions naturally progress vigorously and heal easily; even if exposed to evil winds and foul toxins, they cannot cause harm. Conversely, if Defensive Qi is weak, muscles are fragile, skin is thin, interstices are loose, and pore regulation fails. The body cannot withstand the toxin, allowing it to act recklessly. Thus, the color becomes tender and red, the form flaccid and floating, the pus clear and pale, and the scabs soft and thin. These eruptions break easily and fail to heal. They deteriorate prematurely, without needing external evil winds or foul air. Observing the maturity of eruptions reveals that one must not neglect nourishing the Qi.