害群之马
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。”