People say "One rotten apple can spoil the whole barrel". Rottenness spreads and can do great damage.
In the story told in Joshua chapter 7 we read of a "rotten apple whose name was Achan. He did a wrong thing in hiding away for himself and his family some precious goods from Jericho which God said should have been destroyed. Nobody saw Achan stealing the stuff so he thought he had got away with it. He would have been very pleased with himself and no doubt he and his family made sure the word didn't spread about what he had done. So did his sin keep within the family? No. Someone did see what had happened and this did not just affect Achan and his family it affected all Israel. They failed to capture the little city of Ai and some of their soldiers were killed.
You see God had been watching over his people and he saw Achan sinfully stealing the stuff that everyone knew should be destroyed. God had already told the people that when they sin there are consequences — bad things will happen; there is always a price to pay. Not only does our sin affect ourselves; it can bring down God's judgement on families and communities as well as on ourselves. One rotten apple can create much harm.
Dear Lord, I pray that you will help me not to sin so that others may not be hurt. Amen.