How did Itachi get sick?

[[ As far as I know, we’re never told in canon. 

Given nothing is mentioned about him being sick pre-massacre, we can only assume it’s something that happened during his time in Akatsuki, and rapidly began shortening his life.  There’s a super good theory about what he has, but in the end it’s just a theory. (a game theory….)

Here’s the page I have on it. 

At the best I’d assume it’s the stress that went into working for Akatsuki. He pushed his body too hard. His guilt might have made his body weaker, too. 

I believe in the Jinraiden volume, it’s mentioned that Itachi went to a special village to get his medications. I should read that and see what else about his illness is mentioned. ]]

Sasuke looks at his brother, trying his best to keep his sulking expression. It’s hard when Itachi laughs like that. His brother doesn’t seem to get many chances to laugh. “Plus he kept teasing me.” Sasuke ruffles the back of his messy hair. “Though I have to admit he did teach me…” he says. It’s easier to work through his pride and admit these little secrets to his brother.

“You are the youngest.” Shisui had even teased him from time to time. 

“I’m as strong as I am because of him. Please cherish your time with him.” If Itachi did not have the time to meet Shisui any longer, he absolutely would gladly hand it over to Sasuke. 

“He’s tricky,” Sasuke mutter to his brother. Very clever but he did manage to land a few blows. “Not easy to predict, unlike some of the others. Kind of like you.” Sasuke looks ahead. “Where did you go on your mission? Or is it classified?”

This brings Itachi light laughter. Tricky, and uneasy to predict? That’s what made him annoying? It’s what made him a challenge to Itachi, and he respected it. Shisui was always pushing him further, and he’d like to think he was helping his friend get stronger, too. Sasuke will only accelerate exponentially the longer he trains with Shisui.

“It was simply guard duty for an important leader while he traveled.” It’s not often his missions actually take him out of the village, but even then, they’re rarely that exciting. Quite a lot of them were security detail. 

“I trained with a few of the clan members,” he responds. Sasuke turns to face Itachi. He looks over at his mother for a second, then back at his big brother. “I sparred with Shisui the other day. He’s annoying,” he huffs. Sasuke picks up his tea and wrinkles his nose when the cold liquid touches his lips. “How was your mission?”

Oh? 

Itachi can’t fault him, really. Not with how often he’s gone. It’s only a matter of time before he grew impatient and turned to others. It’s still slightly painful, but it makes sense. It was difficult to  balance his mission load and his desire to spend time with Sasuke.

(It’d be alright in the end, he told himself. It was for Sasuke.)

“The mission was fine. About Shisui…?”

Now this, he’s curious about. Not once did Itachi think that Shisui was annoying, how could Sasuke come to that conclusion?