Revelation 22:2; The Leaves of the Tree

Revelation 22:2 (KJV)  In the midst of the street of it, and on either side of the river, was there the tree of life, which bare twelve manner of fruits, and yielded her fruit every month: and the leaves of the tree were for the healing of the nations. 

The tree yields additional benefit through its leaves – and the leaves of the tree were for the healing of the nations. 

The nations benefit from the health-giving qualities of the leaves (Seiss). Healing does not necessarily indicate the presence of disease. The disease for this this healing provides is that of the former creation which no longer exists in the New Jerusalem. The very next clause in v3 which tells of the absence of the curse demonstrate the impossibility of disease in the new order (Walvoord).

Revelation 22:3 (KJV)  And there shall be no more curse: but the throne of God and of the Lamb shall be in it; and his servants shall serve him: 

Healing, then must connate a promoting of the health of the nations such as will be an ongoing service in the new creation (Govett, Seiss, Walvoord). This agrees with the identification of the nations in Revelation 21:24.

Revelation 21:24 (KJV)  And the nations of them which are saved shall walk in the light of it: and the kings of the earth do bring their glory and honour into it. 

[Robert L. Thomas, Revelation 8-22 – An Exegetical Commentary, Moody, 1995, 484-485]