Nine, Twelve and Five are slowly dying as a result of the experiments decaying their brains.
The symptoms for a intercranial aneurysm include a sudden and unusually severe headache, double vision and a loss of consciousnesses among other symptoms.
Five, Nine and Twelve have all mentioned that they’re running out of time and coupled with the recent clarification of one of the men who worked in the organisation telling Shibazaki that he’s surprised that they’re still alive, but won’t be for much longer, we can assume the worst.
Twelve is most likely asymptomatic meaning he feels no symptoms at all even though the problem is present. Hence his lack of hurry to diffuse all the bombs, he was probably and has been aware and acceptive of his death for some time. He just wanted to comfort Lisa in their final moments. In addition, Twelve is carefully monitoring Nine in order to make sure that he’s fine and tells him to not lie about his condition.