Apparently the virus has mutated and everyone has it now...rrrriiiiiiiight?!
There was a moment at the CDC, season 1, that Dr. Jenner whispered something to Rick... wasn't revealed, what he said to Rick.
Rick hinted about it a few times, like when he was still talking to that family via the walkie-talkie, he says, "a doctor told me....well... that doesn't matter any more..."
Speculation is that Jenner told him everyone is a carrier of the virus. Which would mean anyone who dies will then become a walker, so long as their brain is still intact -- And, that Rick knows and has been sitting on it. He presumably took out the guys in the bar with head-shots, to make sure they didn't come back as walkers.
This could also explain why the military was shooting live-people in the hospital, in the premiere.. these people were already infected, and the soldiers were trying to contain the infection.
But it is kinda contradictory... considering the many non-walking dead bodies we saw in cars on the highway, and the fact that Rick originally wanted to hang the prisoner with a noose.
Possible spoiler: Apparently, in the comic it has ALWAYS been that way, that anyone who dies will become a walker regardless of being bitten.
Seems kind of a stupid choice to make this some kind of surprise-twist, tho.. if Rick knew, what POSSIBLE reason would he have to keep this to himself, and not WARN everyone else?