In Children of Earth, Jack Harkness is blown up and we see body parts put in a body bag, but before the next day is done, he has regenerated and is alive again, even when his body was barely there.
In Miracle Day, he becomes mortal and all other humans on Earth become unable to die. The cause for the "Miracle" was the reversing of a morphic field, which switched mortal to immortal and vice versa.
But many humans were Category 1, those that should be dead. While they continue, in many cases, to move around, many are also comatose or barely alive, yet they don't regenerate, like Jack did. In fact, with the field reversed, humans have a totally different reaction than Jack does to death. Humans simply aren't dying, but Jack (before the reversal) dies, then regenerates.
If it's just a field reversal, why does death effect humans so differently than the way it effected Jack before the field reversal?