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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.

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15y ago

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