It's never explicitly 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.
Notably, the punch snaps him out of it, and he remains reasonable and coherent after that, never lapsing back into paranoia or hostility. He also doesn't have any of the skin lesions the other infected developed over time.
David and Judy appear to be immune, but the people in Cedar Rapids become infected, which would suggest that they're carriers and not truly immune. What on Earth happened to allow them to pass it on?
As pointed by Russel, the toxin is in the water and their home is furthest from the source, so it could simply be good luck.
