Board Thread:Versus Debates/@comment-43403813-20190907005521/@comment-32685487-20190923171051

Novels say he became sick after awakening his MS but before Konoha Crush and for years used medicine to fight the illness so that one day Sasuke could be the one to finish him off. It just got much worse around the actual time of the Naruto series, with him being pretty much nearly dead by the time he fought Sasuke.