in walking dead, people are all already infected.when you die you turn into a zombie(unless you sustained massive brain injury). If you choke to death on a twinkie 15 miles from any zombie, you will come back as a zombie. The bites of zombies are deadly and spreads some poison that kills you. Then when you die you come back as a zombie. So at first they thought the bites turn people into zombies, but then they figured out that wasn't the case.