痘欲其疏，疏则毒少，不欲其密，密则毒甚，此古今确论也。疏密之分，各有
喜忌，如头面欲疏，元首不可犯也，颈项欲疏，管龠不可塞也，胸背欲疏，脏腑俞
募之所附也，若夫手足，不忌其密矣。谓之疏者，非但稀少也，铺排磊落，大小匀
净，亦可以言疏，谓之密者，非必盛多也，攒聚粘连，片复一片，虽只数处，亦可
以言密。疏而凶者亦有数等，如初出时才见红点三两处，其热便退，可以语其疏也，
苟大热不解，唇口燥裂，大小便秘，烦躁不宁，或身无热，但增烦渴者，此由毒甚
郁遏于中，不能遽出，日复渐出，渐加稠密，一也。又如初出一两点，顶尖焦黑，
或三四粒作一堆者，仅见数处，他无所出，喜睡不食，烦渴，大小便涩，此由毒伏
于中，加谵妄者，不待起发而死，若能起发，后必发痈毒也，二也。亦有出现实疏，
遂生玩忽之心，禁忌不守，风寒不避，饮食所伤，汤丸之试，变生不测，三也。设
有密而吉者，治之早，卫之严，里无病，而疮悉成也。
常观朝华之草，夕而零落，松柏之坚，凌冬不衰，夫以草木坚脆不同，坚者难
坏，脆者易伤，况于人质有浓薄，气有强弱邪，彼疮痘之毒，喜老而恶嫩，苍蜡、
娇红，色之老嫩也。紧实、虚浮，形之老嫩也。浓浊、清淡，浆之老嫩也。坚浓、
软薄，痂之老嫩也。老嫩之故，卫气主之，经曰∶卫气者，所以温分肉，充皮肤，
肥腠理，司开阖者也，是故卫气强，则分肉坚，皮肤浓，腠理密，而开阖得也，所
以收敛禁束，制其毒而使不得以放肆，故色苍而蜡，形紧而实，浆浓而浊，痂浓而坚，
自然易壮易靥，虽有邪风秽毒，不能害也。如卫气弱，则分肉脆，皮肤薄，腠理疏，
而开阖失也，所以不胜其毒，而毒得以恣其猖狂之性，故色娇而红，形虚而浮，浆
清而淡，痂软而薄，易破难靥，不待邪风秽气，而先败坏矣。观夫疮之老嫩，则气
不可不养也。
痘疹（天花或麻疹等出疹性传染病）的皮疹希望分布稀疏，稀疏则毒邪较轻；不希望分布密集，密集则毒邪严重，这是古今公认的定论。
疏与密的区分，各有其宜忌。例如头面部希望稀疏，因为头部是诸阳之会，不可受邪气侵犯；颈项部希望稀疏，因为咽喉要道不可被堵塞；胸背部希望稀疏，因为这里是脏腑俞穴和募穴所在之处；至于手足部位，则不忌讳皮疹密集。
所谓“疏”，不仅仅是指数量稀少，如果皮疹分布均匀、颗粒大小一致且洁净，也可以称为疏；所谓“密”，并不一定是指数量极多，如果皮疹聚集粘连、层层重叠，即使只有几处，也可以称为密。
表现为疏但预后凶险的情况也有几种：第一种，刚发出时只见到两三个红点，热度随即消退，看似稀疏，但如果高热不退，嘴唇口腔干燥破裂，大小便不通畅，烦躁不安，或者身体没有发热，只是烦渴加重，这是由于毒邪深重郁结在体内，不能迅速透发出来，日子久了逐渐发出，逐渐变得稠密，这是一类情况。第二种，刚开始发出一两点，顶端焦黑，或者三四粒聚成一堆，仅在少数地方出现，其他地方没有发出，患儿喜欢睡觉且不进食，烦躁口渴，大小便涩滞，这是由于毒邪潜伏在体内，如果出现胡言乱语的症状，甚至不需要等到皮疹起发就会死亡，如果能起发，后来必然会发生痈疽毒疮，这是第二类情况。第三种，虽然皮疹出现得确实稀疏，但家长因此产生轻视疏忽的心理，不遵守禁忌，不躲避风寒，饮食不当受伤，或者随意尝试汤药丸剂，导致病情发生不可预测的变化，这是第三类情况。
假设有皮疹密集但预后吉祥的情况，是因为治疗及时，护理严密，体内没有病变，而所有疮疹都能顺利成熟。
经常看到早晨开花的草，傍晚就凋零零落，而松柏的坚韧，经历寒冬也不衰败。草木的坚脆不同，坚韧的难以损坏，脆弱的容易受伤，何况人的体质有浓薄之分，元气有强弱之别呢？那些疮痘的毒邪，喜欢“老”而厌恶“嫩”。面色苍暗如蜡、娇嫩鲜红，是颜色的老嫩；形态紧实、虚浮，是形状的老嫩；浆液浓浊、清淡，是浆液的老嫩；结痂坚硬浓厚、柔软单薄，是结痂的老嫩。老嫩的原因，由卫气主宰。《内经》说：“卫气，是用来温煦肌肉，充实皮肤，肥厚腠理，主管汗孔开阖的。”所以，卫气强盛，则肌肉坚实，皮肤厚实，腠理致密，开阖功能正常，从而能够收敛束缚，制约毒邪使其无法放肆妄为，因此颜色苍暗如蜡，形态紧实，浆液浓浊，结痂浓厚坚硬，自然容易壮盛并结痂愈合，即使有邪风秽毒侵袭，也不能伤害它。如果卫气虚弱，则肌肉脆弱，皮肤薄，腠理疏松，开阖功能失常，因而不能抵御毒邪，毒邪得以肆意猖狂，因此颜色娇嫩鲜红，形态虚浮，浆液清稀淡白，结痂柔软单薄，容易破溃而难愈合，不需要等待邪风秽气侵袭，就已经先败坏了。观察疮疹的老嫩，就知道元气（正气）不可不养护了。
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.