How can Tree of Life’s leaves heal the nations?

The nations that are mentioned are the earth’s inhabitants that will live during the Millennium. They do not have resurrected bodies; they are mortals. On the other hand during the millennium, the resurrected saints will enjoy the fruit from the Tree of Life, which provides them eternal life. The earth’s inhabitants cannot partake of this fruit, but they will be given access to the leaves from the Tree of Life. These leaves can be used for medicinal purposes. The leaves being used for medicinal purposes, parallels with the manna in the wilderness that fell daily rom heaven. The Children of Israel ate that manna for forty years and they did not suffer from any sicknesses or diseases, Deu. 8:4, “…raiment waxed not old upon thee, neither did thy foot swell, these forty years.”  The leaves from the Tree of Life will be used for medicine for the humans that will live during the Millennium Period