@Invader_Retro@MadParadox earlier it said those infected would still be human with the urge to bite the main artery in another's neck, he wasn't bitten so he's still turning, but she was bitten especially in the neck causing the infection to spread almost instantly like all the other zombie bites.
The scientist explained how the disease works and its original symptoms, which the guy infected was already showing symptoms of before they left the school