1. Proverb It only takes one bad person, thing, element, etc., to ruin the entire group, situation, project, etc. Refers to the fact that a rotting apple can cause other apples in close proximity to begin to rot as well. A: "It used to be considered the top research facility in the region, but after one of its researchers was discovered to have plagiarized a number of his papers, the whole department's reputation has been dragged through the mud." B: "Well, a rotten apple spoils the barrel." I really loved my job, but there was this one jerk there who made life miserable for everyone. One rotten apple spoils the whole barrel, unfortunately.

2. Proverb The criminal, unethical, corrupt, or otherwise negative behavior of a single person will spread to other people around them. Ever since that crook took over as CEO, the company has been backsliding into shadier and shadier business arrangements. A rotten apple really does spoil the barrel. A: "Is it any wonder? When corrupt liars take office, suddenly everyone who benefits from them starts to believe—and spread—their lies to maintain the status quo." B: "It's true, then, that a rotten apple spoils the whole barrel."