Since QM didnt specify what type of zombie will appear that will destroy civilization there are several ways it can happen.
1. If its the slow walking dead kind I expect it would turning millions of people instantly into zombies especially those with weak immune systems like the old and the young children and recently deceased.
2. Rage virus highly virulent and deadly, survivors must not ever go to mainland as it spreads through blood(like a drop of their blood splashing on your eyes can turn you into a zombie) and bites.
3. Cordyceps last of us zombies(forgot how virulent it is).
4. Left 4 dead green virus, highly virulent(rumored to be airborne; some survivors will be carriers of the disease) and mutates very easily and quickly so fast it might as well be made in resident evil, the way it mutates depends on the health of infected if he smokes often then he will become a spitter zombie(there are no radiation zombies YET since in the game the players were never near nuclear power plants or irradiated sites).
5. World war z zombies are capable of having horde intelligence that can pile of one another to overcome walls, run, high speed of infection and turning humans into zombies after 10 seconds after being infected. Are capable of surviving deep underwater/on the sea floor even the ones that have enough pressure that can crush humans like a grape.
6. I am legend zombies where there are 2 types of zombie strains 1 is airborne and the other is not, zombies would have high intelligence and act vampire like and get burned by sunlight.
7. Resident evil zombie with different strains like high mutation like the g virus, the fungi virus is resident evil village, las plagas, the t-virus, etc... low survival rate for us.
8. Cataclysm dark days ahead zombies rob throws us the black goo(instantly turns 99% of humanity into zombies with multiple zombie variants like hulk zombies, and electric zombies), eldritch in origin the goo is sentient and can bring back dead zombies back to life in a few hours despite destroying their brain, the only way to put down a zombie permanently is to pulp their body through blunt objects.
9. Prototype blacklight and redlight zombies basically combine movie "the thing" with zombies. Chance of someone becoming a evolved.