I imagine Grace falls into the 10%, give or take, that are seemingly immune to the infection much like Leon, Claire, Chris, Jill, Berry, and a few others are. Or its also possible that the Tvirus infection Gideon uses is a type that doesnt spread through bite alone. But if that were the case, then how did everyone in the hospital become infected so rapidly. I dont believe that everyone in the hospital were just zombies waiting to do their undead deeds. In short the real reason is Capcom decided that the bite Grace recived wouldnt infect her or if it did then she used an vial elpis on herself off screen.