Disease. I'm still working out the kinks, but essentially it's a virus that works in three stages. Stage one, abnormal behavior and some physical effects. Stage two, seizures followed by a coma. Stage three, zombieism. The bodies are still alive, however the infected are at this point mindless, driven by the need to consume (anything, really) and infect. They just go until they drop. You could call it decay, I suppose. The infected basically rot away until they die, theoretically having served their purpose of infecting further hosts.

I'm not entirely sure whether it's man-made, natural, or some combination. I'm leaning towards man-made via standard disease routes (pigs, I'm thinking, but not sure yet).