Chinese medicine answers the question of why people fall ill with a pair of terms rather than with a single cause: Zheng Qi, translated variously as upright or righteous Qi, and Xie Qi, the pathogenic influence. Upright Qi is not a substance. It is the body's working capacity to hold itself together, to warm and move things, to close a wound, to keep an invasion out and to settle back afterwards. A pathogen is anything that pushes against that capacity.
The two sentences quoted everywhere are these: "When upright Qi is present within, the pathogen cannot intrude" (Su Wen, on needling methods), and "Where the pathogen gathers, the Qi is necessarily deficient" (Su Wen, on evaluating heat diseases). Read together they say that illness is a relation between two things, not the arrival of one.
Notice what this framing is not. It is not germ theory. The six influences — wind, cold, summer heat, dampness, dryness and fire — are named after weather, but "wind" here describes how a set of findings behaves: sudden onset, shifting location, upper part of the body. It does not claim there was something in the air. A person who develops a stiff neck after sitting in a draught is not being told that the draught contained a substance.
Nor does "deficient" correspond to any measured value. No instrument reports upright Qi, and the word cannot tell you who will catch an infection and who will not.
What survives translation into modern terms is the narrower claim underneath: the same exposure makes some people sick and leaves others well. That much is real, and research into how much of an agent actually reaches a person, into immunity and into host factors takes it seriously. What the classical sentence does not give you is which factor it is for any particular person.
This matters because "deficiency" is a comfortable word. Calling a persistent problem "just deficiency" is one of the ways an examination gets delayed. Unexplained weight loss, drenching night sweats, blood in the stool or urine, a new lump, or a fever that will not settle are not matters for this vocabulary. They need an investigation, and no pattern name substitutes for one.
This entry describes how the literature frames illness. Whether supporting upright Qi changes the course of any named disease has not been established.
中医对「为什么会生病」的回答不是给出一个病因，而是给出一对词：正气与邪气。正气不是某种物质，而是身体自我维持的能力——温煦、推动、固摄、防御、痊愈，以及事后恢复平稳的能力。邪气则是任何与之相抗的作用。
被引得最多的两句是：「正气存内，邪不可干」（《素问·刺法论》）与「邪之所凑，其气必虚」（《素问·评热病论》）。两句合起来说的不是某样东西来了，而是两者之间的相对关系。
要留意这个框架「不是」什么：它不是病原学说。六淫以气候命名，但「风」描述的是一组表现的行为方式——起病急、部位游走、多在上部，而不是说空气里含有某物。迎风之后项强的人，得到的并不是「风里有东西」这个判断。
「虚」也不对应任何可测数值：没有仪器会给出正气值，这个词并不能判断谁会被感染、谁不会。
能真正翻译到现代语境的是它下面那句更克制的主张：同样的暴露，有人发病、有人不发病。这一点是真实的，接触量、免疫与宿主因素的研究正是围绕它展开的；但古典那句话并没有告诉你是哪一个因素在具体这个人身上起作用。
这一点很重要，因为「虚」是个让人安心的词。把迁延不愈的问题说成「只是有点虚」，是检查被推迟的常见方式之一。不明原因的体重下降、大汗盗汗、大小便带血、新出现的肿块、持续不退的发热，不属于这套词汇的处理范围——它们需要检查，任何证名都不能替代检查。
本条只描述文献如何影响疾病这件事。至于「扶正」能否改变某个具体疾病的病程，尚未被证实。
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