They are! It's likely they'd be immune, though you could have them change into a different sort of zombie if they got infected if you'd like. Like, if they came in as a boomer from Left 4 Dead and got bitten and turned into a completely different sort of zombie, as an example. Or they could be bitten and just be able to survive the effects better than most people, etc.

...Most of the zombies in this world would probably just think of them as their own, if you didn't want to go that route. Zombros.