Pestilential qi (戾氣)
Wu Youxing's term for a harmful qi, different from any weather, which he held caused epidemics. It entered through the mouth and nose and settled between the surface of the body and the inner organs.
The Origin of Epidemics
Wu Youxing, Treatise on Pestilence (Wenyi lun), 1642, opening section · 1642
傷寒與中暑感天地之常氣疫者感天地之厲氣在歳運有多寡在方隅有厚薄在四時有盛衰此氣之來無論老少强弱觸之者即病
Cold Damage and heatstroke come from the regular qi of Heaven and Earth. An epidemic comes from the pestilential qi of Heaven and Earth. From year to year there is more or less of it; from place to place it is thicker or thinner; through the four seasons it rises and falls. When this qi comes, old or young, strong or weak, whoever touches it falls ill at once.
疫邪與瘧彷彿但瘧不𫝊胃惟疫乃𫝊胃始則皆先凛凛惡寒既而發熱又非若傷寒發熱而兼惡寒也
Epidemic qi is much like malaria, except that malaria does not pass into the stomach, and only an epidemic does. Both begin with shivering and a dread of cold, and then fever follows. This is not like Cold Damage, where fever and the dread of cold come together.