Review Detail of SEBnator in Reincarnated as a Dog with System

It's a very interesting Storry to characters are good the world seems exciting and I can't wait to read more. I only have one problem about the world I don't really understand. It seems to me at first that the mana zombies where create in that instance when the people also got their abilities for example the one in the staircase. But in the news and in the telephone talk with the health center semms everything fine so far and all where good but suddenly like out of nowhere almost everyone is a mana Zombie and all those people that where fine at first and even had abilities suddenly turn also into zombies. I mean it is possible but from the explanation that a mana zombie is a humen who couldn't withstand the mana and turned into a zombie shouldn't all zombies be created at the exact moment when mana first arrived like in the night wenn they got their abilities?