Allies (and you, with the Player Infection mode) can get infected during combat, and then need daily antibiotics to keep the Infection from claiming their lives: if they're without them for three days, they die. (Most survivors will ask you to put them down before they turn, but some will take the initiative to "solve the problem" on their own, so to speak.)
Of course, this is not the only way to die in Dead State. The undead could eat them, other survivors could shoot them, and they could even commit suicide or get murdered by another ally.
The moment you start to like an ally might just be the moment before a bullet hits them right in the head, so watch out.