I think one of the big reasons for the relative lack of immune individuals in either game, other than how frequent the factors that confer immunity (Or at less asymptomatic expression, as Devlin still had the virus in his body but his immune system was capable of suppressing it.) would be the fact that the only way to know that you were resistant, or immune to the virus would involve being bitten or exposed to bodily fluids of an infected ‘Zombie’.
And given that most people, including the person who was bitten, would know such a thing is a death sentence (The General Idea of if you’re bit you will turn into a monster), I could very well see most immune individuals killing themselves to ironically avoid turning, or being shot by other survivors if they (the Person bitten) were unwilling to end themselves, as you wouldn’t want a newly turned Zombie charging you when your guard was down. And that’s assuming that the immune individual was able to survive the confrontation with the zombie that lead to them being bitten, and given that you would only be in a situation to be bitten when your for lack of a better word losing a fight, its likely quiet a few people die straight off the bat.
Plus, one would have to remember that there would probably be lots desperate, or just plain stupid people who believe they are immune, and say as such, or think a shot of anti-biotics will keep them from turning, only for surprise surprise they turn.