Fridge Brilliance / Fridge Horror: It's never explicitely clear that Russel does in fact become infected with the crazies. His nosebleed appears immediately after he gets punched in the face. Otherwise, all of his symptoms could be explained by him simply cracking under the pressure of the day. This would make his Heroic Sacrifice rather darker and sadder.
And Becca shows all the signs of having the sickness (at the stage she would have it) up until she's killed.