actually it might genetic defect. there are genetic defects that won't appear until puberty or later on in life (in this case the 1st time itachi awakened ms)
cant be called a plot no jutsu disease as the disease had no significance in the manga....no significance = presence or absence of disease doesn't affect the result.
he wanted to die by sasuke's hands anyway, but yes, he had to take great dose of medicines to be able to live upto the when sasuke came for revenge.