Apple is a very common used symbolic artifact in various literatures, films and culture. It can represent various things, including fertility; love; wisdom; divination; knowledge; luxury; deceitfulness and death.
Rotten apple, usually means a bad individual among many good ones, and especially one that spoils the group.
For example, rotten apple was used as the title of TIMES magazine's headline one day, pointing out that it is nearly impossible to fire a teacher under the protection of teacher tenure. The article tells the story of the bunch of Silicon Valley guys want to change the current situation of teacher tenure.
Rotten apple, usually means a bad individual among many good ones, and especially one that spoils the group.
For example, rotten apple was used as the title of TIMES magazine's headline one day, pointing out that it is nearly impossible to fire a teacher under the protection of teacher tenure. The article tells the story of the bunch of Silicon Valley guys want to change the current situation of teacher tenure.