This is all from the best of my memory so please excuse me if you run into a couple of eggshells.
For those who have faithfully watched The Walking Dead from beginning to current, we are all aware of Rick in hospital. We are also aware that Shane believed his best friend was dead as this was his heartfelt defense to Lori. He tried to evacuate the coma-induced Rick to save him from the walkers infesting the hospital. He laid his ear to Rick’s chest to hear or see any sign of life because the machine completely stopped functioning from a sudden quake [ helicopters, Humvees, loud booms, etc. a war zone ]. No heartbeat. “Give me a sign, any kind of sign, to let me know you’re alive.” Nothing but commotion drawing nearer and nearer, Shane blocks the exterior door with a bed and dashes towards the exit.
At the Center for Disease Control now run by Dr. Edwin Jenner alone, an odd interaction happens: Jenner whispers into Rick’s ear. Earlier, Jenner had a mandatory rule that everyone would need to get a blood test. Fast forward it’s discovered that everyone’s infected; when you die, either by walkers or old age, you turn. Shane’s confirmation of this.
Now if all these are added up, could it be he is infected like the rest but if he were to get bit, his infected blood cells would behave as anti-biotics?
Rick’s infected and if he were to get bit, his physical appearance would show some signs of corruption? Just like Michonne chained up two walkers as her pets, Rick’s corruption would be a camouflage; walkers would obviously sense him as one of their own [ recall Glenn and Rick covered themselves with walker organs on their way to get an escape van? ].
Perhaps Rick’s infected and the general rule applies to him as much as the next person, but upon death he resurrects as half-human half-walker? A Rickracula; tempted to feed but his human reasoning tries its best to resist.
How about Rick’s not infected at all? He entered the world immune. Does this mean his family’s exempt? Cameras didn’t show Lori a walker or give any hint if a transformation ever occurred. Carl had to do what had to be done, regardless.
Let’s say Rick was dead at hospital like Shane claimed. For how long was he gone, then? Assuming he was the only human inside the hospital, Rick must have miraculously jumped start his own heart while pronounced dead. But this seems far fetched. It was learned that turning into a walker takes minutes to hours [ don’t exactly remember the numbers at the moment but I want to say 3 minutes to eight hours ] in the first season.
Something unique must have had happened before this fiasco; maybe a nurse accidentally spilt milk into his IV bag? Maybe red wine was mistaken as donor’s blood? If the source can be known, then probably it’s the cure. There’s an open possibility of no cure at all.