etc. He who touches pitch must expect to be defiled. A rotten apple will injure its companions. One scabby sheep will infect a whole flock.
To the same effect is the locution, “C'est une brébis galeuse,” and the idea implied is, he must be separated from the flock, or else he will contaminate others.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894