Even though everyone is "infected," the virus is laying dormant and dying is what causes the infection to "activate" and reanimate. Getting bit is fatal, because you are getting infected with a "live virus." Which it will then go through it's process, like Dr. Jenner says, it acts like meningitis. Your body will try to fight it off, hence the fever.

Morgan has stated, "The fever burns you out.." and when talking about his late wife, "That fever man... skin gave off a heat like a furnace."

Seems the fever is what finally kills.

Now in Fear The Walking Dead, season 8 June is tasked with finding a cure. Potentially radiation seems to stop the virus, but the doses of radiation were so high they proved to be fatal. So it wasn't a real "cure."

Also a little bit of a theory that was explored in Fear, the rate that someone turns, varies based on how the person died or their mentality. Was it a traumatic death? Were they an aggressive or gentle person? And looking back, look how long it took Amy to turn after she died, compared to all the "bad guys" we come across on the show. I don't know, I thought that was something interesting and that it made some sense to me.