bad apple

From the proverb “one bad apple can spoil the whole bunch” (or a variant thereof).

• "Near-synonym: bad egg"
• "There is one bad apple, and that is Katie, the beautiful prostitute with whom Catcher Bruce is in love. Unlike the cliche harlot of fiction, she is as short of compassion as Bruce is of IQ."
• "While most union leaders are people of integrity, there are still bad apples."
• "If anyone still harbors the fantasy that the business scandals of the past few years were the handiwork of just a few bad apples, they should read John Bogle's "Battle for the Soul of Capitalism.""

• rotten apple
• bad seed
• bad egg

• there's a rotten apple in every barrel
• rotten to the core