一粒老鼠屎坏了一锅粥

yī lì lǎo shǔ shǐ huài le yī guō zhōuyililaoshushihuaileyiguozhou

1. lit. one pellet of rat feces spoiled the whole pot of congee (idiom)
2. fig. one bad apple can spoil the whole bunch

Study it in Yanamino