One scabbed (sickly) sheep infects a whole flock.

　　The rotten apple injures its neighbour.

　　Bad egg of the community.

　　eg：That naughty boy is the black sheep of the family.

　　中英俚语俗语来源：庄子：“夫为天下者，亦莫以异乎牧马者哉?亦去其害马者而已矣。”