In The Walking Dead we see thousands of walkers. Is the infection that causes this just in Atlanta, all of America, or is it the whole world? I feel like they don't make that clear in the show or the comics.
The Walking Dead plays a few months AFTER the infection started. There is a prelude series called "Fear the Walking Dead" where it is shown that the infection started slow, but managed to spread so fast that within days entire cities and the entire US was more or less overrun. It is indicated a few times in both shows that it is NOT a US only phenomenon but world wide, but as far as I know it is never stated explicitly.
On the show, we know that the infection has spread as far as Europe, with a heavy implication that it is worldwide (which is probably the case anyway by the same reasoning as the comics). From "TS-19" (1x06):
JENNER: It was the French. [...] They were the last ones to hold out as far as I know. While our people were bolting out the doors and committing suicide in the hallways, they stayed in the labs till the end.
The creator has said that future shows set on other continents are possible, lending further credence to the idea that the plague of walkers is worldwide:
When asked whether we might see more companion shows set on other continents like Europe or Asia, Kirkman demurred, "If I did [know of plans for that], I certainly couldn't say that here," he teased. "But the success of 'Fear' shows that there's certainly some appetite for that."
In the comics, there is not definitive proof of what is happening in the rest of the world. Indeed, the comic's creator is on record as saying that he purposely is trying to leave out information about the scope of the epidemic:
"I probably would have changed that stuff," he says. "I've been careful in the comic series to not say what's happening in other parts of the world. It's something that's going to be fun to explore in the spinoff series. But the fact that France is mentioned in that episode and other things like that, I probably would have steered away from that stuff if I had to do it all over again."
However, there is good reason to suspect a worldwide epidemic in the comics.
Over the course of the series, the characters notice no:
- Foreign countries sending aid to survivors
- Planes flying over that would indicate air travel has survived.
- Radio broadcasts received from foreign networks.
and perhaps most important
- massive international military efforts to destroy all the walkers.
It is very possible that the ocean is not an effective long-term barrier to walkers, if they could travel across the bottom or float, which makes the survival of other countries even less likely.
More places that have been stated or implied to be overrun:
Cynthiana, Kentucky (Rick's hometown in the comics)
Philadelphia, Pennsylvania (Dave and Tony met each other fleeing Philadelphia)
Dave: We met on I-95 coming out of Philly.
Tony: Damn shit-show that was.
- The Walking Dead, S2E8, Nebraska
New York, New York (Philadelphia is only 75 miles from NYC, so if New York was safe, we can assume Dave and Tony would have headed there instead of Georgia, which is 700 miles from Philadelphia)
Jacksonville, Florida (Tyreese's hometown)
Dallas, Texas (Rosita's hometown)
Houston, Texas (Abraham's hometown)
Savannah, Georgia (video games)
Barcelona, Spain (setting of the one-off comic, The Walking Dead: The Alien)
Fort Lauderdale, Florida (hometown of Kenny in the video games)
Wellington, Ohio (in the video games)
Las Vegas, Nevada (where Christa and Omid from the video game were when the outbreak began)
San Francisco, California (Christa and Omid's hometown)
North Carolina (video games)
Tennessee (video games)
Maryland (video games)
Western Virginia (video games)
Macon, Georgia (video games and television show)
However, it is pretty clear that the infection has spread worldwide. That is the only way to explain Jenner's statement here:
Dr. Jenner: There is no hope. There never was.
Rick: There's always hope. Maybe it won't be you, maybe not here, but somebody somewhere...
Andrea: What part of "everything is gone" do you not understand?
Dr. Jenner: Listen to your friend. She gets it. This is what takes us down. This is our extinction event.
- The Walking Dead, S1E6, TS-19
If the epidemic was limited to the US, or the US and Europe, or the US, Mexico, Canada, and Europe, it would be terrible, but it wouldn't make an epidemiologist say that the human race would soon be extinct. Jenner would only call the outbreak an "extinction event" if the whole world was being consumed.
All we know is that everything from Georgia to Washington is gone, and although it hasn't been dealt with at any length, the clear assumption on the part of the characters is that the crisis is so widespread that they couldn't possibly get to a place that wasn't infested with zombies.
However, it is telling that in the time that has elapsed over the course of the series - at least 3-4 years - there have been no signs of life on any continent. Not a single airplane has flown overhead - and if the outbreak were limited to North America, one would expect that other countries would continue flying planes, even if it were just surveillance flights monitoring the outbreak.
And issue #10's publicity blurb read:
After the ordeal Rick has endured last issue, he sets out to find safer shelter. More is learned about the zombies that now out-number us 5000 to 1, but when it comes to some things, it's better not to know. Rick begins to wonder if there is a light at the end of the tunnel his life has become. Even if there is, how can he ever expect to make it there?
- Image Comics
Whether "us" means "American humans" or "humans" is up for debate, but I suspect the latter.
The Walking Dead and Fear the Walking Dead share the same universe and (roughly) the same timeline; Fear the Walking Dead: Flight 462 was a miniseries that led into FTWD.
The Walking Dead:
In the first season's finale, TS-19, Dr. Jenner at the CDC reveals that Europe is gone:
Jenner: It was the French.
Jenner: They were the last ones to hold out as far as I know. While our people were bolting out the doors and committing suicide in the hallways, they stayed in the labs till the end. They thought they were close to a solution.
Jacqui: What happened?
Jenner: The same thing that’s happening here. No power grid. Ran out of juice.
Fear the Walking Dead and Flight 462:
Flight 462 attempts to land in Phoenix, Arizona:
The protagonists of FTWD start out in Los Angeles:
Then they get on a boat and head for San Diego:
Travis: Nick found a logbook on that boat. We got a problem. San Diego's dead.
Madison: How bad?
Travis: It's gone.
Madison: What? Wait, wait, wait.
Daniel: What do you mean gone?
Travis: It's burned. The military burned it down. According to this, the Leigh Anne just came from the south right there.
They reach an island off the coast, called Catalina:
The ranger at Catalina has some bad news:
George: You folks are from Los Angeles. You got that air about you.
Travis: We left when they started bombing.
George: Napalm. It's what they're using up and down the coast. I've been checking in with some of the other ranger stations.
Travis: It's in every city?
George: As far as I can tell. Portland, Seattle, Vancouver.
Travis: And south?
George: San Diego was burned.
Travis: (sighs) Further south?
George: They shut down the border. No way in or out.
Travis: Oh, God. So what's left?
George: Not a hell of a lot. Interior is no better than the coast. Last time I connected to Joshua Tree, ranger said Petrified Forest, gone. JT shut down soon after. That's Cali, that's Arizona. Zion went dark before them. Utah gone. Rocky Mountain station is dead. That's Colorado. That's the Continental Divide. That's a good goddamn half the country.
- Fear the Walking Dead, S2E2, We All Fall Down
Everything west of the Rocky Mountains, from Canada to the US/Mexico border:
The protagonists get to Mexico:
Cities, states, provinces, and countries said or implied to be infected:
- Atlanta, Georgia
- Basically, everywhere else in Georgia
- Philadelphia, PA
- New York, NY
- Washington, DC
- Richmond, VA
- Blue Mountains, VA
- Alexandria, VA
- Basically, the rest of Virginia
- Parker's Crossroads, Tennessee
- The rest of Tennessee
- North Carolina
- Dallas, TX
- Houston, TX
- Fort Lauderdale, FL
- Jacksonville, FL
- Cynthiana, KY
- Las Vegas, NV
- Columbus, OH
- Wellington, OH
- Los Angeles, CA
- San Francisco, CA
- San Diego, CA
- Joshua Tree, CA
- Catalina Island, CA
- The rest of California
- Zion National Park, Utah
- The rest of Utah
- Rocky Mountain National Park, CO
- The rest of Colorado
- Petrified Forest, AZ
- Phoenix, AZ
- The rest of Arizona
- Portland, OR
- Seattle, WA
- The rest of the US
- Baja, Mexico
- Vancouver, BC, Canada
- Barcelona, Spain
The shows make it explicitly clear that the outbreak spreads far beyond the shows' locations - at the very least, the entire US, and at least parts of Mexico, Canada, and Europe, are gone. It is extremely likely, however, that the entire world has been consumed.
The comics have revealed less, but there is absolutely no reason to believe that there are any unaffected places left in the world.
In no Walking Dead media has there ever been any indication - airplanes, radio transmissions, etc. - that any continent, country, state, province, county, or city is any better off than the locations we have actually seen. By all appearances, the entire world is gone.
In Season 3, Episode 8 of "Fear The Walking Dead", Victor strand finds the Abigail run aground. The short wave radio picks up a signal from a survivor. The survivor happens to be a cosmonaut. The cosmonaut states "I watched the world go dark".
Its unlikely the infection could remain on only one continent.
I think it started in Los Angles, then spread across the US. After which it spread to Canada, then Mexico, then all of South Amercia, then Alaska. Then it hit Russia, then Africa, then Great Britan, then Australia, then Spain, then Japan, then Brazil, then China, then France, then Italy, then Greece, then Turkey, then the Ukraine, then Saudi Arabia, then India, then Papua New Guinea, then Mongolia, then Iceland, then Greenland, then Norway, then Sweden, then Finland, then Greenland, and then spread worldwide.