Board Thread:Naruto Discussions/@comment-3965545-20160202201023/@comment-142748-20160202201619

Since Itachi had disappeared for quite a long time and he thought Danzo was going to betray the deal they made. Hell it wouldn't be surprising if Danzo didn't know his wearabouts since Itachi directly reported to Hiruzen in secret. So with Hiruzen dead, his protection over Sasuke lifted, Danzo was in prime position to eliminate Sasuke.

Thekillman wrote: We don't know the nature of his disease, it's possible he had it for years before that but didn't show symptoms until much later. And its also equally possible Itachi developed his disease very recently.