I don't know about flies and maggots; but, this season's Walking Dead has some pretty gnarly looking zombies. I noticed it right off this season, that the zombies were looking worn out and nasty. At one point, a zombie brushes against a car and the skin on it's face just sluffs off.
I think that they realized that it has been a while since the whole apocalypse started and, as you indicate, the zombies should start looking pretty bad. So, this season the zombies are looking pretty nasty.
I have bad news for you. The Zombie problem is here and now. Just look at the University of Missouri and a few other places.