When you're down, negative emotions find it easiest to sneak in.
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history
From the Song Taoist encyclopedia 《云笈七签》 by Zhang Junfang: 自后垣乘虚而入,径及庭中 - he entered through an unguarded rear wall and went straight to the courtyard. The phrase moved from military contexts into business and emotional metaphors.
usage
Common in military, sports, business, and health writing (e.g. 病毒乘虚而入 - viruses exploit a weakened immune system). Often paired with a cause such as 防守松懈 or 免疫力下降.