Kokichi and Kirumi knew about Kaito's illness. Kaito being how he was, wanted no one to worry, and made Kirumi promise to not tell anyone(Dumbass). Kirumi has tried to convince Kaito to tell the others, but failed. The 3rd(and final) time, Kirumi took no for an answer

Follow-up! You guys remember what Kokichi did about him dying? He walked into the bathroom to clean that crap off, and found Kaito coughing up blood...And they just stood there for a solid minute...Before Kaito threatened Kokichi to keep his mouth shut, and surprisingly he did keep shut