Since Christians still get sick, isn't this proof that we are still suffering from the effects of the "fall" and from our sinfulness?
Here is my brief response:
With regard to "sickness" being the result of sin: It is true that "spiritual sickness" is the result of sin, or perhaps we should say that those terms are synonymous. But in the New Heavens and New Earth, ie the New Jerusalem, ie Heaven, no one is *ever* sick:
Isaiah 33: 24 And the inhabitant shall not say, I am sick: the people that dwell therein shall be forgiven their iniquity.
Sickness ended with the forgiveness of sin. That is why the psalmist says that He heals all our diseases:
Psalm 103: 2 Bless the LORD, O my soul, and forget not all his benefits: 3 Who forgiveth all thine iniquities; who healeth all thy diseases;
If the benefits of salvation (the forgiveness of sin) are that all of our diseases have been healed, and we are never sick, then we cannot say that physical sickness is caused by sin. Anymore than we can say that the physical planet is "fallen" from the the "good" creation because of sin. (Otherwise we would be looking for a future redemption of it, thereby denying the cross has already accomplished this.)
I think our bodies get sick because they are frail, decaying, and as ordained by the natural created order, they will return to dust.
(I know there are passages which speak of physical sickness as chastisements, or discipline from the Lord toward His children, but that is a separate issue and not the same thing as attributing sickness in general to a "fallen" nature--which has been resurrected to new life in Christ; or to "sin"--which for God's people has been forgiven once and for all.)