The zombies are all sheep! And when they 'turn' a human, it turns into a weresheep-zom ... dammit, that's been done.
OT: Okay, for real this time. The zombies are just people. The infection causes parts of their flesh to only affect light outside the visible spectrum, giving the illusion of missing flesh ... but here's the kicker, a zombie doesn't see the visible spectrum, they actually see the spectrum in which the entirety of the zombie is visible. So to zombies, zombies look like people and people look like oddly-coloured zombie-things, and to people, zombies look like zombies, and people look like people. And the final kicker, apart from the change in eye-sight and skin colour-stuff, there is no difference between a zombie and a human, they even maintain their sense of self when they change. And at no point in the movie does a zombie attack a human, it is always defensive actions, and the minor cuts don't spread the infection, because the infection is air-borne, and through a combination of incubation times and the fact that the infection needs an open wound to spread, it just appear that the zombies are causing the spread.
That's right, it is a movie about racism.