Scientifically speaking, a particularly virulent pathogen could completely span the globe in a little as a week. If released in an airport of any major city with international travel, it could conceivably have been on every continent within a week and spread widely by the end of a month depending on its virulence and contagion level. The more contagious, the faster it spread. The more vectors, the more likely it spread. If it was a coordinated attack, it could have been placed at every major airport ensuring its spread in as little as 72 hours.
For the Walking Dead to make sense, we have to make certain suppositions:
- The virus is likely a bio-engineered technology created by human beings. Judging from its behavior, it was designed to be a disruptive agent placed within an enemy population to destabilize the infrastructure of the target society. Designed to remain invisible until activated by an external force. Judging from Dr. Jenner's response, it was something completely new and unheard of before he began to work on it.
For some real world perspective: Dutch Researcher Created A Super-Influenza Virus with the Potential to Kill Millions
As a hidden agent, it would need to be completely invisible but able to be spread by contact or by aerosol dispersion. Disguised as a common influenza virus it could be spread by animals (think Bird Flu) or even people by coughing like any other influenza. In similar movies like the Crazies, the agent would have first been spread by plane and then later by contact with water supplies.
Since we don't know much of the history of the world before the outbreak, we cannot be sure if there was a massive influenza pandemic before the apocalypse occurred but if I were betting, I would assume the people responsible may have released it during that pandemic as a cover if the disease had very noticeable symptoms.
Here's where we run into problems. If it was asymptomatic, (meaning no one noticed they were infected), it might be more difficult to spread. It might require a deliver system like a water facility or an aerosol sprayer. This would make it difficult to spread easily.
If it was transferred by Patient Zero (first person believed infected during a pandemic, sought because they may reveal the original source of a natural pandemic) it would not have been able to spread as far asymptomatically without someone noticing it during testing. There have been people who were carriers of diseases without suffering from them, Typhoid Mary, for example.
This means the virus had to have been spread quickly, silently, artificially and at a pandemic speed for everyone, everywhere to have been infected without anyone being aware of it, which lends credence to the idea it was a bioweapon, spread intentionally or unintentionally is anyone's guess. Judging by the completeness of its distribution, I would assume terrorism as an intentional delivery.
There may be other activation factors but touching the infected zombies, being splashed by their blood (assuming you aren't bathing in it) wearing their entrails as a disguise (didn't seem to cause anyone any difficulties in the early episodes) you will live to be a ripe old age, assuming one of your chronic infirmities (high blood pressure, diabetes), other diseases, or crazed friends don't get you caught and eaten first.