It only takes one 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, just one bad apple spoils the barrel." I really loved my job, but there was this one jerk there who made life miserable for everyone. One bad apple spoils the whole barrel, unfortunately.