And first he has to have a way. Ask the doctor. If your body was perfect and like God made it in the beginning, it would be impossible to get sick. But the devil has to find a weakness somewhere to pervert. 
That's the same thing with your soul. Reason you don't believe in Divine healing, the devil found a weak spot in you, and he perverted it. God is the Healer. And we're offsprings of God, sons and daughters of God believing it like our Father believes.