Ever since Rick's coma, I've wondered "Is this all a dream?". Maybe this is like a Wizard of Oz sort of thing (minus the ruby slippers and all that stuff). I have two predictions going.
a) That Rick will wake up in the hospital, everything is alright. There's a TV in the room announcing that a virus has just broken out. It'd be pretty cool if that's how it ended, I think. But, it might not be the ending...because that'd be a really complex dream but whatever.
b) The virus was man-made and it got out of hand. Rick will die and Carl will be left with other survivors, and must recreate society.
So, those are my thoughts.