The title "The Walking Dead" doesn't refer to the zombies. It refers to Rick's band of survivors. So trying to expand on the zombie plague, or fit it into a real-science framework, or anything similar, misses the point.
Missing the point is a series that (like every AMC series) prides itself on being so "real," yet do not follow their own back-patting in the scripts. Again,
no one said the show was about the cure (and I find it sort of interesting/telling that some of you are going overboard to defend the suggestion of characters behaving realistically in even
discussing it).
Then again, this is the same series where characters--in soap opera fashion--tolerate the intolerable from others for incredibly long periods of time, such as Andrea and the Governor (not killing him when anyone else would, and still defending him against Michonne). Or how about
anyone not killing Shane when his every waking moment in season 2 was being an instigating, threatening ass. Yeah, if your behavior toward your so-called best friend/"brother" is more hostile/threatening than not (and having a good idea about the Lori matter's implications), how safe are the others he was not close to? Yeah.
In reality, people are killed for less, but this series tries to play the "real" card where human behavior is concerned...selectively.