Board Thread:Naruto Discussions/@comment-3965545-20160202201023/@comment-24165784-20160202222133

Itachi's Sickness Was Explained In The Naruto Jinraiden Novel To Be Caused By Itachi Awakening His Mangekyou Sharingan, As A Rare Side Effect Of Said Awakening Happening. Itachi Had Eye Drops To Subside It For A Long While, But He Had An Incomplete Version Of The Medicine, So It Wasn't Cured. Sasuke Almost Had The Same Sickness, But He Got The Completed Version Of The Medicine Shortly Before His MS Was Awakened Fully (Or Maybe It Was Shortly After, I Forget). But It Was Stated That Itachi Using His Mangekyou Made His Condition Worse Along With The Disease.