What if Danzo poisined Itachi?
I mean, we know that he snatched Shisuis eye like it was nothing, that man has no respect for Uchihas at all.
What if he saw Itachi as a threat, perhaps for saving and protecting Sasuke.
So he got some poison maybe from Oro, perhaps the same poison that possibly killed Hashirama.
And so He made Itachi sick with decease he couldnt cure, something that was slowly killing him. Danzo is not afraid of sacrificing others for what he "believes" is the greated good. I dont think he liked letting Itachi live just like that.
Or maybe it was carma since Itachi killed so many uchihas it made him deadly sick.
Even if he is trash maybe he sneaked up on him or some.
Shisui was probably stronget then Danzo but he managed anyway, maybe Itachi couldnt avoid it cus he wouldnt fight konoha shinobi or some.