rotten apple spoils the barrel, a

Prov. A bad person influences everyone he or she comes into contact with, making them bad too. Helen is the rotten apple that spoils the barrel in our office. Everyone sees her come in late to work and take long coffee breaks, and they think, "Why can't I do the same?"

One bad individual can spoil an entire group. The idea was stated as long ago as the fourteenth century (in a Latin proverb sometimes translated as “the rotten apple injures its neighbors”), long before the mechanism of spreading mold or other plant disease was understood. Benjamin Franklin repeated the sentiment in Poor Richard’s Almanack (1736): “The rotten apple spoils his companion.”