I think this is what is happening in the show right? They get sick, die, then reanimate.
In the prison yes. They didn't really get into the flu origins, if it's a new flu strain that's a mega flu because of all the decomposing diseased flesh walking around, or if every Zombie with bleeding eyes was a living human who died of the flu.
That's the only way the flu would really work.
Almost nothing on the show regarding zombies is possible if you scientifically examine it deeply enough, so I am sticking to a nice "it's an allegory" analysis and let the hard science go in exchange for a morality play, that set in this world to give us something as powerful as anything I've seen in literature or film.
Exactly, people need to just throw everything aside and take it in for what it is....a good hour of tv enjoyment. If you get that out of it , isn't it all worth it?