Well, let's say 1/3 have some immunity or resistance. They still have to run away from the other 2/3s. Whatever the turning is, it has low penetrance. There are no where near 5,000,000 million biters running around LA, unless they all ran somewhere to hide and started digging to get out of sight.

I toss out the toxoplasmosis as an example in real life that infects human and affects behavior. We still have a really small sample size. Three people choke, two do not. The two that do not have genetics, exposure to a recently turned and being a vet. I put Saul in the vet, because I got the feeling He spent a lot of time there.