I know it is not a story, but I have always wondered.
Because the show is set in North America, that is what I will work with. And assume the rest of the world matches.
The population of the lower 48 states of the United States is approximately 329 million.
If one percent did not become zombies that leaves 3.29 million survivors. That number seems high but there are over 19,000 incorporated cities or towns in the US that averages 173 people per town.
If 1 in every 10 survivors was a fighter that would give us 329,000 zombie killers across the country.
If each of these killers only killed one zombie a day for a year the total would become 120 million dead zombies, a year.
Using that basic math, there would be no zombies left in the United States after three years.
My question is, after more than five years where do all these hoards of zombies still come from?