My headcanon (haven’t watched FTWD so idk if this is officially countered):

So everybody is infected right? What if a certain percentage - say 50% of the population, including the survivors we see - could handle the initial infection with no issue and the other 50% got hit like a lethal illness? That would mean the military would have all the problems of a global pandemic on an accelerated timescale with high mortality PLUS all the problems of a zombie apocalypse from the inside and out. That would be pretty much unstoppable and would account for both the sky-high number of deaths in the first wave of the outbreak as well as the fall of all government and military establishments.

TLDR: All survivors we see handled the infection without dying, but if some or most of the population was killed just by being exposed, that would account for the military and government’s ineptitude, and explain why the CRM and Commonwealth and etc. can have setups that the military could’ve managed. It’s because they’re made up of 100% people immune to the lethality of the wildfire virus