Carl Has Three Chances To Live
The grave you saw Rick at in the preview for next season could be Gabriel's. He was sweating a lot in the last season just like Carl and might have been bit prior. So, Gabriel might go and the helicopter we saw this season in an earlier episode rescues Carl where they are working on a cure and we are left in limbo not knowing his fate. Probably a LONG shot but possible.
Two: another possibility is that he is immune to the virus.
Three: another possibility I read is that he was bit by a whisperer and not a walker.