I may be splitting hairs here, but there is a difference between being "infected" and being symptomatic.
One can be infected with something which can lie dormant and not show the ultimate result for a considerable period of time (think HIV).
In this case, the catalyst for transformation is death. Now we know that walker bites are lethal - lead to the fever, death, eventual turn.
However, one could be infected and just carry the. . .whatever it is.
I seriously doubt that we will ever get an effective explanation for what this is. Of course, we logical folks will continue to look for one, so let's carry on with the theorizing!