one bad or rotten apple can spoil the whole barrel

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• the rotten apple injures its neighbour — The proverb is also found in a number of variant forms, some of which are illustrated below. Cf. L. pomum compunctum cito corrumpit sibi junctum, a rotten apple quickly infects its neighbour. 1340 Ayenbite of Inwit (EETS) 205 A roted eppel amang… …   Proverbs new dictionary