http://www.npr.org/2011/05/09/136017612/bad-apple-proverbs-t...

>Back then, nobody ever talked about "just a few bad apples" or "only a few rotten apples" — the whole point was that even one was enough to taint the group. These days, those are the phrases people use to imply that some misdeeds were an isolated incident — a couple of rogue cops, a handful of unprincipled loan officers, two or three sociopathic soldiers. Then there's the version that goes, "There are always going to be a few bad apples." That's a counsel of moral realism: as in, there's evil in the world; get over it. It's not a sentiment you would have heard in a 19th century sermon, much less from a grocer you were complaining to about the wormy fruit he'd sold you the week before. "Well, Mrs. Gold, we all have to expect find a few rotten apples, don't we?"