The Walking Dead fans are nothing if not creative and there’ve been a fair few widely-discussed theories over the years. Unfortunately, though, a lot of them have been proven untrue as the franchise has developed. For instance, the idea that it was going to be revealed that old Carl was telling the whole story at the end was obviously ruled out when Rick’s son died. But given that the Brave Man himself still survives somewhere out there, another old theory is still feasible: that Rick is immortal.
There are two parts to this concept. One is that Rick is immune to the walker virus. Now, this definitely isn’t true in the comics and has to date been untested in the TV universe, too. There’s a chance that Andrew Lincoln’s upcoming movie trilogy will explore efforts to create a zombie cure though, so maybe Rick’s blood will prove to be the key or something? Still, there’s too much conjecture to put too much stock in this one right now.
The second part questions why the outbreak started in the first place. What if it was man-made, following experiments to cheat death and make human beings immortal? Obviously, these experiments backfired and the intent was corrupted, as people did survive death, but they became mindless monstrous, cannibals. That said, what if the intended effect worked on a select few and they became immortal? A select few including Rick Grimes.
For evidence that Rick could be immortal, just look at any of the times he’s survived near-fatal injuries. He’s been shot, impaled, blown up, etc. You name it, he’s walked away from it. Obviously, this is because he’s the protagonist and he has to survive, but it would certainly be a neat twist if The Walking Dead writers were able to come up with a reason for Rick’s super-durability that turns everything we know about the zombie virus on its head.