But from what the CDC guy said and what has been show, it is actual death and decomposition.
Not something that Hershel was ever told, however. This was one of the bigger oversights for me in how the group behaved (not that they're known for their foresight or competence).
Why would he have to be told? The evidence before his own eyes (and nose) would seem pretty convincing. Why would Herschel think that he could "restore" a body with tissues in that condition any more than he could for one who had died the old-fashioned way? I can't think of any reason except he's not thinking rationally on that point.