One bad apple can damage the morale of the whole team.
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memory
Literally: a horse that harms the herd. Picture one sick horse spreading disease to a whole stable — the idiom captures how one bad person can corrupt an entire group.
history
This expression originates from the Zhuangzi (庄子), the classical Daoist text. The original passage warns that even a single harmful element in a group can destroy the whole.
