The only other thing that I'm thinking is it could be passed through blood. Shane has obviously had zombie blood on him and was stabbed with a knife that had been used to kill zombies. Daryl had shoved a knife into Randalls leg and who knows what knife they were using when he first hurt it.
This might explain why the two guys at the bar that were shot didn't come back as zombies while they were having the shoot out since Shane regenerated really quickly and so did Randall.
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