Just pulling in one of my early observations. The first zombies are described as covered in Scars and Bruises by Michael. The general populace is not covered in scars. This implies repeated damage and healing as if subjected to experimentation

Whatever it is at ground zero, it gets past standard gas mask protection, unless CJ sabotaged that also. Assumption, whatever is at ground zero it is at a very high concentration and it only takes a little to have an effect. This is an Airborn contagion and really bad news.

In chapter One Riley does say the closer they where when asked how long did it take for people to change. CJ also says "if they get close" when she does not want to have her leg amputated. This is a bit of a reach, but I have felt that the zombies themselves have an area of affect in addition to the bite or scratch. Maybe just sweat or water vapor from respiration.

Assuming you are right about the little ones coming from inside the Earth and a subterranean ape theory. One of the things they might have is extra thick finger nails to dig. Going further with the Ant-Ape, the majority of the Ants in the colony are underground, Saul asks, where did everybody go? Something else, animals in the dark can have light-blindness. Just sit in the dark for an hour and turn the lights on.

Victor has been exposed twice and Pegs once. Did they not get a high enough dose or do they have their own partial immunity, kind of like a higher tolerance?