Apple in barrel

A commonly used cliche refers to "one bad (or rotten) apple in a barrel".

It is currently often used to imply that an observed fault is exceptional.

I believe that this is wrong, and that the original idea is that a single bad apple can cause the rest to go rotten and should therefore be removed immediately on discovery.

I would like to know whether I am right about this or, if not, what the real meaning is.

I would also like to know the whole wording of the saying and its origin.

You are correct in that a single bad apple could cause the rest to spoil and the bad apple should be removed or quarantined ASAP.

The traditional version is "One bad apple spoils the whole barrel." It means that one bad member can corrupt a group. I'm not familiar with the "exceptional fault" interpretation you describe; this must have resulted from a misunderstanding of the proverb.

The "exceptional fault" interpretation seems to be widely used by spokespersons on US television talk shows. A cop or pol is accused of something and a spokesperson for the police department or the political party protests that to the affect that there will always be a "few bad apples" so don't pass judgment on the department or party (Who really should be vetting their persons.). There may be a difference between the "one bad apple" phrase and the "few bad apples" excuses.