yin and yang (陰陽)
Paired forces, such as dark and light or cold and heat. Scholars and doctors alike held that when they left their proper places, or the seasons came out of order, sickness followed.
Preface to the Treatise on Cold Damage
Zhang Zhongjing, Treatise on Cold Damage and Miscellaneous Diseases, preface · c. 200–206 (within ten years of the start of the Jian'an era, 196)
夫天布五行以運萬類人禀五常以有五藏經絡府俞隂陽㑹通玄冥幽㣲變化難極
Heaven spreads out the Five Phases to keep all things moving. People receive the five constants, and so they have the five organs. The channels and the acupoints, and the ways yin and yang meet and flow together, are dark, deep, and subtle, and their changes are almost impossible to fathom.
Discourse on Epidemic Qi, and a Brother's Letter
Cao Zhi and Cao Pi, quoted in the Taiping yulan (Imperial Readings of the Taiping Era), chapter 742 · 217 or soon after (the essay, on the epidemic of Jian'an 22); 218 (Cao Pi's letter)
此乃隂陽失位寒暑錯時是故生疫
What happened is this: yin and yang had left their proper places, and cold and heat had come at the wrong times. That is why the epidemic arose.