They list only two method of virus spread, but seem to leave out a third; zombification/infection by both blood AND by latent, airborne contact.
**This will be kinda spoilery, but mostly open knowledge.**
In this model, "zombies" are created not just by blood contact, but by an airborne pathogen. So the initial wave of zombies were created due to a spore/fungus. One that was based on a real fungus so I would think that would rank even higher than say, "Shuan of the Dead".
So the ways to contract this "disease" are many:
1) The typical bite, scratch, splatter, etc due to blood contact
2) The fungus/spore that started the whole thing existing in the wild
3) Those infected, once they reach a specific point, either by the host being used up by the fungus, or by some external death, create a landmine for an airborne version
That third point is very important. Generally, once you "kill" a zombie, the threat is neutralized. Just kick 'em to the curb and burn when you get a chance. However in this model the corpse is still a carrier and must be handled and disposed of carefully to prevent new contamination. Once the fungus reaches a point where the host body no longer sustains it, it starts releasing spores which can infect any passer-by. So even though you stopped the horde today, tomorrow the battlefield can become one giant infectious cloud.
Which means that while the population takes up arms to stop the physically attacking bodies, you need to dedicate a large percentage of the population for waste disposal. You're not going to be an effective fighter wearing a hazmat suit so the two groups should not mix. This depletes the number of people "fighting the disease" which may allow for greater rates of infection.