Thank you for the memories The Walking Dead signature shirt. Answer contains spoilers if you haven't watched The Walking Dead. In the TV series, there is no info, so far, on how the epidemic started. At the end of season one, the group we are following arrives at a CDC complex in Atlanta. They find out from the doctor there that this is all caused by a virus which was named "Wildfire." Everyone is now infected with the virus, so that anyone who dies will come back as a walker (unless they are killed by a head injury, or of course killed by someone after they come back).
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