exactly i thought the exact same thing when i first saw it LOL i was like... OFMOGOIWTOWOGOG! ITACHI PULLED A KIMIMARU! : ( LOL heartattack instead of disease though but that doesnt matter
as zetsu said sansasoo (sp) had an effect on itachi. if you noticed as each level of ms jutsu he used hurt him more. only makes sense the most powerful one would have the greatest effect