[Rias]

Originally Posted by LKK

I agree with Jan-Poo. I don't remember Kai mentioning that he was also sick when he described Aki's illness. Furthermore, based on his confusion during episode 3, I don't think he had a clue that he might be ill as well. I don't think he understood what was happening to him then.

I do think he's been ill all this time without his knowing it though. I think his illness explains why he's so good at the game. Remember, he's good enough that other players think he's using some cheat. But I suspect that until now, the illness hasn't presented any noticeably odd symptoms like the double-vision he experienced. In other words, Kai's illness has finally progressed to the point where he noticed for the first time that something could be wrong with him.

Kai being good at games (especially at kill-ballad) has nothing to do with the condition. Later episodes should explain why he's good at games, why FrauKojirou thinks he's a cheater, and why FrauKojirou provided him with the source code so he can use the same system to control a hobby robot.

Spoiler for Condition explanation timing:

He's actually had it the same time that Aki had it. In the VN, it was explained that he had it when Aki had the symptoms showing (same time as episode 2), and the condition of how it works for him was explained when he had it activated during the final fight.