From what I remember of the L4D-style infected, most people first come down with a serious fever/illness that slowly turns them into the crazies that run around after survivors attempt to pull their limbs off.
The survivors in the games were the people who were, for some reason, immune to the full effects of the infection; they were still carriers and capable of spreading the illness, but they didn't turn into full-out Infected like the majority of the population.
But yeah, LET'S BUILD SOME TENSION.