Fniff said:
Well, In most scenarios (At least the ones the survivor guys are basing on) the virus comes out of nowhere. You can't immunize that fast! And plus, most doctors would be eaten alive or insane! Probably without a clue what is going on!
No, mate, you're missing my point.
Immunity doesn't just occur because of vacines. Some people, at least in a species with a nice, healthy genetic diversity(so screw you, eugenicists), will usuaully have a natural immunity, or at least an immune system strong enough to eventually give them one.
IIRC, the survivors in L4D are all meant to be naturally immune.