Well, that was the thing...he never really looked sick. That's why he was able to hide it from the rest of us for so long. I think even Momota-kun himself didn't know about it, in the beginning.

[Shuichi still isn't even sure what he actually HAD. If things had gone according to script, they were supposed to assume he had the meteor space plague or whatever but...that was all fake. So why WAS Kaito so sick? Was that something from his previous life, or was that something Team Danganronpa gave him for the sake of keeping the plot interesting?]