I have often wondered this too. In the scene where
Riley and
Angel were immersed in a mountain of rotting flesh at
the arena I wondered how it was that they didn't catch any kind of infection, especially when Riley got rotting filth in her eye. I'm thinking two things are happening; 1)somehow, the corpses that are being used as food for the zeds are not turning. Could it have somthing to do with how Riley noticed the zeds were drooling like dogs when they smelled sweat? Do they emit a zombie venom of sorts? It would mean they could turn you when they want rather than whenever you contact them. 2) the zombie virus exists in the zeds blood itself. So even if a zed doesn't bite you with a venomous bite, his blood gets in you and you contract the infection, like
tommy did. If the corpses at the arena that Riley and Angel were buried in were victims of the zeds, they could have been infection free based on my notion that zeds emit venomous bites only when they
want to turn you.
So a zombie bites you with it's drooling bite and you contract the virus. But maybe he can turn off his virus spreading capabilities? It's a stretch. But contact with zombie blood will most likely turn you 100% of the time. The corpses at the arena were human victims who were infection free, while the corpses
outside the arena were infected.