[TV] The Walking Dead - Page 213

We haven't actually seen that many zombies in terms of zombie-to-pre-apocalypse ratio. There have never been more than a couple hundred zombies in one place (and the big one was in Atlanta) which is not huge given the millions of people living there before. That being said, either zombies congregate in large groups and these survivors have terrible terrible luck, or there are a shit-ton of zombies. Maybe it's reasonable compared to the original population, maybe its not, Remember though, that we also don't know anything about the genesis or early spread of the virus since Rick was unconscious for it, so its conceivable that there's an explanation there.

And finally, anyone who get bitten or scratched or otherwise comes into close contact with zombies becomes a zombie in turn. Thats a lot of potential for spreading the virus in the first few days before people realize. If you're in a group of 10 people and one gets scratched but you save him, you think everythings fine. That night, the zombie attacks and in the dark, confusing close combat that follows, several more group members are infected. Now thats maybe 30 or 40 percent infected in that group. That would provide a reasonably large supply of zombies if it happened even 1% of the time. And since they seem to travel in herds, it makes sense to some extent that herds would congregate where there are survivors, and survivors tend to go towards the same places (town centers with supplies, pharmacies, etc).