MINI SPOILER. DO NOT READ ANY FURTHER IF YOU HAVE NOT WATCHED THE SEASON FOUR OPENER. So I guess that means anyone who has survived so far didn't have any type of immunity to the virus that's responsible for the zombie outbreak or obviously it's starting to mutate?
They tried that. But, the virus was already inside every human on earth. To wipe it out means wiping mankind from the planet. This isn't Romeo's zombies. Although a walker bite on it's own can kill, you no longer have to be bitten to turn. You just have to die by any means and next thing you know, you're eating that hot blond from the apartment 2 doors down. And not in the way you'd hoped, either. Pretty smart way to address one of the biggest flaws in the zombie mythology, I think.
The three Webisodes have nothing to do with what went on in the opener. The webisodes usually are stories that happen to other people not involved with the main characters or the show. The only thing to connect the show in the recent three webisodes that I posted was it took place prior to the first season in the very hospital that Rick Grimes was in a coma at the time & the message about not opening the door to the room filled with zombies that the guy wrote was the very same door/message that Rick saw when he woke up from his coma & was wandering through the hospital trying to figure out what the hell had happened while he was in a coma.
A great episode! Tonight's episode taught me two things: 1) a katana is better than a .45 and 2) the zombie apocalypse is the wrong place to establish a sense of adolescent independence.