A while back I read a book about these not humans but they live in trees. There is a disease that is the problem. At the end, it gets released into the ocean that is at the center of the world and the trees grow out of it. The characters spend a long time there.
Midworld by Alan Dean Foster ?
From Wikipedia: Midworld is a planet entirely covered by a rain forest three-quarters of a kilometer (almost half a mile) tall. Born is a member of the primitive human society that has lived peacefully on Midworld for hundreds of years, careful not to disturb the natural balance of the jungle.
(Can't see anything about a disease, and I never got round to reading the copy I've had for over 30 years)