Disease and sin are two sides of the same coin.

Bless the Lord, O my soul, And forget not all His benefits: Who forgives all your iniquities, Who heals all your diseases.
~ Psalm 103:2-3

Disease and sin are spoken of as equals in this passage. Sin is to the soul what sickness is to the body. When Jesus died on the cross, He purchased our salvation which includes healing (see Isaiah 53:4-5). There is still a struggle with sin, and there is still a struggle with sickness. Neither one has to be the case. Some say, "If healing is for today, why isn't everyone getting healed?" The proper biblical response would be, "If Jesus has given us a new nature, why do we still sin." The truth is that we don't have to sin if we don't want to because Jesus has made us victorious over sin. The same is true with sickness. We don't have to tolerate it if we don't want to because Jesus purchased our healing. We don't have to be afraid to believe.