I just started rewatching TWD last night...the first person to tell Rick what was going on is Morgan, told him if he gets bitten or scratched, he'll get a fever, die then turn...that's what happened to Morgans wife. Later when Rick first encounters the group in Atlanta, they are smearing walker blood on their clothes to fake out walkers, he tells them not to get blood on skin or eyes...much later they figure out everyone is infected with a virus and you turn when you die even if it doesn't involve walker bite.