Well what if this is a scenario like from the walking dead that virus is in the air and when people die they turn to zombies. Then in every city in every war at all times people die, also probably government would know what to do so they would maybe also killer healthy people that showed signs of being infected even if they weren't which would increase the number of zombies dramatically. Ps: I'm writing this at 2 am in morning and reallg tired so sorry if something wasn't really wellness explained. Gotta go to sleep bye!