In the TV series "The Walking Dead," the characters inhabit a world overrun by zombies - specifically, zombies caused by a mysterious virus that has apparently infected everyone in the population. The living keep the virus in check by unknown means. But when someone dies - whether quickly after being bitten by a "walker" or felled by a human nemesis or more slowly due to natural causes - the result is the same: After death, everyone is reanimated as a bloodthirsty zombie. The pandemic survivors come to realize that the zombies aren't their only enemies: The living can be even more dangerous than the undead. The group of main characters has spent several seasons searching for a safe haven against both walkers and unsavory living people. The beginning of Season 4 finds them ensconced in a prison, the series regulars merged with new characters from the nearby town of Woodbury.
The premise that all living people are already infected with the zombie virus was introduced in Season 1, during a foray to the Centers for Disease Control and Prevention, where the lone remaining CDC scientist - Edwin Jenner - informed Rick, the de facto leader of the survivors, of this fact. (However, Rick didn't tell the rest of the group - and thus the audience - this particular tidbit of information until the end of Season 2.) To date, this revelation has been the central plot point, in terms of public health. Unlike the movie version of "World War Z," there has been little attention paid to how this zombie pandemic began or how to end it - the focus is on daily survival. Understanding the virus probably wouldn't matter much at this point in any case - mathematical modeling has shown that, unless you hit hard and hit early, there's pretty much no stopping a zombie pandemic once it starts. Add on the fact that, based on the survivors' CDC experience, all research into the cause (and therefore any cure, treatment, or prevention) has stopped . . . well, the ragtag group appears to be on its own.