wont the maggots who eat the zombies turn into zombie maggots, and indeed zombie flies. That would increase dispersal beyond anything we could handle, assuming the flies didn't have to actually bite you to transmit. Flies regurgitate their food, it's how they digest. A little scrape combined with a zombie fly and you would be turned in short order. Once zomnbie flies they would also lose interest in the already infected. Zombies only eat the living.