The KJV and the NJKV and the NASB translate the word sarz in Greek as flesh. The NIV translates sarz as sinful nature. Both translations have merit. But in order to avoid all sin with my body I must submit it entirely to the service of Christ. And the reason is that remaining sin is dangerous.
Sin is like the poison of a mamba snake.
It's exceedingly deadly. Every sin if permitted will become imperious in its demands and every lust will aim at its maximum expression. Sin is like the Devil its originator. It is limitless in its capacity for evil. Every unclean thought or glance would be adultery if it could, every thought of unbelief would be atheism if allowed to develop.