What is itachi's disease?

Answer 1

Itachi had an unknown illness that weakened him. Whatever illness this was, it was worsened by his abilities to a point where he could no longer sustain life (by using Mangekyo Sharingan and Susano'o). Before battling Sasuke, he also had been drained from giving a power to Naruto, making him exceptionally weaker for that battle. There are many theories about the illness he had, ranging from lung cancer to Tuberculosis, and even the chance it could be some random unknown illness caused by Susano'o or from his bloodline, or even from depression. He contracted this illness at some time after killing the Uchiha Clan.

Answer 2

I assume your onabout Itachi from Naruto

Every time Itachi uses Mangekyo Sharingan it deteriorates (makes it worse) his eyesight.

The only cure to it, is to take the eyes of another person (brother or sister) who can use Sharingan and replace their own eyes with the other persons eyes and then the eyesight will never worsen.

---Spoiler--

Itachi tries to take Sauske's eyes to replace his own with, but ends up losing his life to sauske and sauske ends up with Itachi's eyes and gains the unlimited use of Mangekyo Sharingan

--Spoiler--

Sasuke doesn't gain (Unlimited use) ETERNAL MANGEKYOU. He just unlocks Mangekyou Sharingan, and Itachi implants HIS Mangekyou techniques into him for protection against Madara.

---Unkown Actually---Spoiler---

For the actual question: It is most likely just the consequence of his Mangekyu leading towards darkness as it was also happening to Madara (as shown by Itachi) until he took his brothers eyes. Although, Itachi might have been suffering from another illness (but nothing of such is mentioned in the manga).

To the former person's statement of Sasukes Mangekyu: I believe it still remains unkown. Itachi did implant techniques into Sasuke, but if you recall, he is also able to use Itachis Mangekyu so Itachi could have implanted it as well (it didnt have to be physicaly like Madara). There is an ocassion where Madara asks Sasuke if he was going to implant Itachis eye, but it seems to me that he already posses it, not only that but he first opens Itachi's Mangekyu which was self-activate and then he opens his (like a premature opening of his brothers eye first) so im fairly sure something new is coming up, that's why Sasukes Mangekyu looks rather odd and unique. The former remark is right though, it is not an eternal as far as we know, until it is told that sasuke is becoming blind will we know that it was just his Mangekyu that he opened with the power to open his brothers Mangekyu as well and not an Eternal Mangekyu.