There seems to be some concensus on the web that the shisper may have been the doctor telling Rick that everybody was infected anyways and just didn't know it. From what I understand, in the comic, Rick digs up Shane or something of the sort to confirm it (although in the comic he wasn't told this by a doctor in the CDC).

I find that a pretty good guess and it would make sense considering we haven't really seen anybody die in the TV series who wasn't bitten and thus their brain destroyed which seems to be the only way to kill a walker. So, if we get our first "natural" death and see Rick start freaking out then I guess we'll know. But, also, if this is what Rick knew then why wouldn't he be really tore up about it and especially about the baby, knowing that the baby would be doomed no matter what. Just thought I'd throw that in.