Those that died in the highway episode died from a virus, the 1st virus, before being affected by the 2nd virus. The "zombie" virus (ie the 2nd virus) is the cure, released by the CDC, to combat the 1st virus. Those infected by the 2nd virus first are surviving and only affected by the 1st and 2nd virus mixing after they die.
So 3 groups of people:
People that died from virus 1 before getting virus 2.
People that got virus 1 first then virus 2 before they died (zombies).
People that got virus 2 first, then exposed to virus 1 (survivors or future zombies).
So it's very simple!