Disease and sickness are rare in the world of Naruto, especially when it comes to important characters. Few are ever affected by a serious illness. Instead, most of those who die, do so in combat or through self-sacrifice. However, Itachi Uchiha notably suffered from a terminal ailment, which raises a lot of questions.
First and foremost, why would Itachi, of all people, be afflicted with a disease that plays such a major role in his death? Itachi's illness may serve a larger purpose regarding the plot, and his character. But what sickness did Itachi have, and how did it manifest?
Stress and Anxiety Worsened Itachi's Illness
Itachi often chooses to take on burdens so that others don't have to. For example, he assumes the burdens of being an older brother, a ninja, a spy, a traitor and a martyr, all in hope of sparing other people from pain. Itachi single-handedly attempts to save his brother, and the entire village.
All of those responsibilities create a lot of stress and anxiety that wear on the body. Itachi himself even acknowledges he attempted to do too many things on his own. When he is revived during the Fourth Shinobi War, he warns Naruto that attempting to do everything alone will lead to failure: "No matter how strong you become, don't try to bear the weight by yourself. You will fail. Don't forget about your friends."
What Disease Did Itachi Suffer From?
For almost his entire existence, Itachi had to live a double life. He was a spy for the Leaf Village while pretending to be a spy for the Uchiha clan. He feigned being a traitor to the Leaf Village while undercover in the Akatsuki. Itachi also had to pretend to be a monster who killed his own family for power when, in reality, he loved them dearly. Itachi was never able to be his true self. Hiding his true intentions for most of his life added to his stress. It was only after his resurrection that Itachi was finally able to be who he truly was.
All this stress, anxiety and turmoil was never openly expressed. Even from a young age, Itachi never showed his emotions or allowed people to get close. This is shown when Itachi used to tap Sasuke on the forehead, creating distance between him and his brother. The emotional pain that came from killing his family, hurting Sasuke and lying about his true identity was never released in a healthy manner. He couldn't contain his suppressed emotions forever, so they manifested as Itachi's illness. Everything he wanted to express, but couldn't, was turned into a type of cancer that slowly ate away at him until his death.
During the Fourth Shinobi War, Itachi admits he views his entire life as a failure. Not being honest with Sasuke about the Uchiha massacre was a mistake; he believes if he hadn't tried to bear all of his burdens alone, everything would have turned out for the better. Tragically, if Itachi had lived his life honestly, and not tried to hide his true feelings, it's possible he would have never contracted his illness in the first place.