Matthew 7:18
18 A good tree cannot bear bad fruit, and a bad tree cannot bear good fruit.
Symbolically it means God wants us to reform. I heard from Asmongold that if you mess someone up mentally, the tree will get bent. Which is a good way to put it.
But… if you took it literally. What would you tell a kid who ate rotten apple on the ground? 
It does strengthen your immune system, even if its not good to do when you get older.