In the first season Shane says that Rick died and had no heart beat. That in itself would qualify him as "dead."
So how is it that Rick has not turned into a walker, while
Shane and Randall both died of non-bite- or scratch-related deaths and both turned to Walkers.
I thought it could have been that it wasn't airborne yet, but Jenner told Rick about it in the first season. Jenner also mentioned he was put down there in the beginning of the outbreak, so if he knows it’s airborne, then it must have been since the beginning.
So is Rick immune or the key to a cure, or am I overthinking it?