I am afraid that attempting to explain this using real world science will always fail. The idea that zombies can be caused by viruses is completely absurd despite it's popularity. A virus [needs living cells][1] in order to be able to reproduce. Therefore, if the host cells have stopped reproducing (as is the case upon death), the virus will be completely incapable of doing anything at all.
I might be willing to suspend my disbelief to accept, for example, bacteria or any other pathogen doing this but a virus is just silly. Pop-science just happens to find viruses _cooler_ is all. 

A dormant virus is simply one that is not currently hijacking the host's cellular machinery to make more copies of itself. OK, that explains the survivors. However, having the virus activated by death is, while theoretically possible (see, I can too suspend my disbelief!), very futile. The virus will wake up, try to replicate itself and fail because the cells are dead. 


Since we know that zombies do not heal, we also know that their cells are dead despite their tissue being animate which means that a virus would simply not function. Therefore, all bets are off when it comes to explaining this using actual science. We have to posit that viruses are a completely different thing in -universe to what they are outside it and all "scientific" discussion is moot.




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Full disclosure: I'm a biologist so I tend to take umbrage at such blatant disregard for fundamental scientific facts :)

  [1]: https://en.wikipedia.org/wiki/Viral_replication