Throughout the series, Itachi is seen coughing up blood. Did Itachi have a disease?
Yes, Itachi did have a disease. Its specifics are never stated, but Itachi fought against the illness with some type of medicine to complete his plan on Sasuke. Information on the disease is scarce.
The original Naruto manga gives small details to the disease, outlined in chapter #401. It’s explained to Sasuke that his brother was “ravaged by illness, painfully aware of his impending death,” and he “forcefully prolonged his life with medication.”
Itachi’s sickness was not introduced until late into the story. It was implied during his fight with Sasuke that his issues were related to overuse of the Mangekyo Sharingan. However, the later clarification of his illness causing impending doom implies its unrelated to jutsu.
Why did Itachi die so easily?
Itachi died easily because he allowed Sasuke to defeat him. Itachi’s long term plan involved developing Sasuke’s Mangekyo Sharingan by letting him win the fight. By joining the Akatsuki, Itachi also set up Sasuke as a hero who protected the Leaf Village. Some reputation was restored to the Uchiha clan.
Although Sasuke was at his most powerful during their fight, it’s likely Itachi was still holding back. Itachi has fought, killed, or escaped from dozens of high ranking and notorious ninja. Fighting someone who possesses an inferior version of sharingan, Itachi could have beat Sasuke if he really wanted to.
What if Itachi was never sick?
If Itachi was never sick, it was likely things would have gone the same. Itachi was aware Sasuke’s search for him would never cease. The sickness encouraged the two to meet quickly.. He took “medication” to prolong his life in order to fulfill his plan on Sasuke. Itachi felt it was best to sacrifice himself in order to bestow new powers upon Sasuke while repairing the Uchiha name.
Itachi did have a disease. The specifics were never explained, but it required him taking medicine to prolong his life.